test(i18n): derive the plural-branch guard instead of listing the keys

The guard walked a hand-written array of 23 keys, so a key given a plural
or select branch after that array was written was checked by nobody — it
had drifted four behind (AppTabsNotRestored, LaunchWorkspacesLeftRunning,
SearchTabs, SftpReplaceBody). This is the same hand-maintained-copy
problem the key list itself was rid of, and the same answer: walk
`L10nKey::ALL` and treat any key the English catalogue gives a branch to
as one every locale must spell out.

All four turned out to be translated, so nothing was broken for a reader;
what was broken was the check. Deleting zh's "one" branch for one of them
now fails the test, where before it passed.

The smoke pass also runs in each supported locale rather than only the
one the test happened to be left in.

src/ui/i18n/mod.rs

@@ -1713,63 +1713,54 @@ mod tests {
 
     #[test]
     fn plural_and_select_branches_are_translated() {
-        let plural_keys = [
-            L10nKey::SettingsDeleteProfileCascade,
-            L10nKey::SettingsAliasesLinked,
-            L10nKey::SettingsImportSummary,
-            L10nKey::SettingsImportIgnored,
-            L10nKey::SettingsRulesOpenedWithConnection,
-            L10nKey::SettingsOfflineMachines,
-            L10nKey::SettingsForgetPasswordSharedBody,
-            L10nKey::PanelMoreChangedFiles,
-            L10nKey::ScmFilesChanged,
-            L10nKey::ScmStagedFileCount,
-            L10nKey::WindowStopShells,
-            L10nKey::WindowDeleteShells,
-            L10nKey::DiffChangedFiles,
-            L10nKey::DiffUntrackedCount,
-            L10nKey::DiffMoreFiles,
-            L10nKey::DiffUntrackedHeader,
-            L10nKey::DiffMoreUntracked,
-            L10nKey::DiffUntrackedSummary,
-            L10nKey::HomeTimeMinutesAgo,
-            L10nKey::HomeTimeHoursAgo,
-            L10nKey::HomeTimeDaysAgo,
-            L10nKey::HomeTimeWeeksAgo,
-            L10nKey::HomeTimeMonthsAgo,
-        ];
-        for key in plural_keys {
+        // Derived from the English catalogue, never hand-listed: a key given a
+        // plural or select branch is picked up here the moment it exists. The
+        // list this used to carry had drifted four keys behind.
+        let mut checked = 0;
+        for &key in L10nKey::ALL {
+            let mut has_variants = false;
             for branch in ["zero", "one", "other"] {
-                assert!(
-                    !translate_variant(0, key, branch).is_empty(),
-                    "missing en plural/select branch {branch:?} for {key:?}"
-                );
-                assert!(
-                    !translate_variant(1, key, branch).is_empty(),
-                    "missing zh plural/select branch {branch:?} for {key:?}"
-                );
-                assert!(
-                    !translate_variant(2, key, branch).is_empty(),
-                    "missing ja plural/select branch {branch:?} for {key:?}"
-                );
-                // Not every key spells out a "zero" branch; the ones that do
-                // must spell it out in every language rather than lean on the
-                // English fallback.
-                if translate_variant_en(key, branch).is_some() {
+                let Some(en) = translate_variant_en(key, branch) else {
+                    continue;
+                };
+                has_variants = true;
+                assert!(!en.is_empty(), "empty en branch {branch:?} for {key:?}");
+                // The fallback in `translate_variant` would quietly serve the
+                // English to a reader who asked for another language, so a
+                // branch English spells out must be spelled out everywhere.
+                for (name, text) in [
+                    ("zh", translate_variant_zh(key, branch)),
+                    ("ja", translate_variant_ja(key, branch)),
+                ] {
+                    let text = text.unwrap_or_else(|| {
+                        panic!("{name} is missing plural/select branch {branch:?} for {key:?}")
+                    });
                     assert!(
-                        translate_variant_zh(key, branch).is_some(),
-                        "zh is missing plural/select branch {branch:?} for {key:?}"
-                    );
-                    assert!(
-                        translate_variant_ja(key, branch).is_some(),
-                        "ja is missing plural/select branch {branch:?} for {key:?}"
+                        !text.is_empty(),
+                        "empty {name} branch {branch:?} for {key:?}"
                     );
                 }
             }
-            // Smoke-check t_plural does not produce empty strings.
-            assert!(!t_plural(key, 0, &[]).is_empty());
-            assert!(!t_plural(key, 1, &[]).is_empty());
-            assert!(!t_plural(key, 5, &[]).is_empty());
+            if has_variants {
+                checked += 1;
+                // `t_plural` picks the branch; none of the counts may land on
+                // a hole.
+                for n in [0, 1, 5] {
+                    for lang in SUPPORTED_LANGUAGES {
+                        set_locale(lang.code);
+                        assert!(
+                            !t_plural(key, n, &[]).is_empty(),
+                            "{key:?} renders empty at count {n} in {}",
+                            lang.code
+                        );
+                    }
+                }
+                set_locale(default_language_code());
+            }
         }
+        assert!(
+            checked > 20,
+            "only {checked} keys have branches — did the catalogue move?"
+        );
     }
 }