test(ssh-config): let the second write to an included file be seen as one

`an_alias_added_to_an_included_file_is_seen_without_touching_the_root`
failed on every Windows run of this branch and had passed on main by a
margin the branch happened to spend elsewhere.

The cache keys on mtime, the test writes `conf.d/work` twice back to
back, and Windows stamps files from a clock that ticks about every 15ms —
so both writes can land on one mtime, and the cache is then correct to say
nothing changed. Nothing in the product depends on resolving edits that
close: the poll behind this runs at 4Hz and a real edit arrives at human
speed. It is the test that is too fast, so the second write now says when
it happened rather than sleeping out the tick.

src/core/ssh_config.rs

@@ -1423,6 +1423,7 @@ mod tests {
         );
 
         std::fs::write(ssh.join("conf.d/work"), "Host work\n").unwrap();
+        edited_just_now(&ssh.join("conf.d/work"));
         assert!(
             cache.resolves(&cfg, &root, "work"),
             "an alias put back in an included file is found again, \
@@ -1430,6 +1431,21 @@ mod tests {
         );
     }
 
+    /// Make a write that has just happened look like one, on a filesystem
+    /// whose clock is coarser than this test is fast.
+    ///
+    /// Windows stamps files from a clock that ticks about every 15ms, so two
+    /// writes as close together as the ones above can share an mtime — and a
+    /// cache keyed on mtime is then right to say nothing changed. A real edit
+    /// arrives at human speed, and the poll behind this cache runs at 4Hz, so
+    /// the collision is an artefact of the test rather than something a user
+    /// can reach. Stated by hand instead of slept out.
+    fn edited_just_now(path: &Path) {
+        let f = std::fs::File::options().write(true).open(path).unwrap();
+        let ahead = std::time::SystemTime::now() + std::time::Duration::from_secs(1);
+        f.set_modified(ahead).unwrap();
+    }
+
     fn temp_root(name: &str) -> PathBuf {
         let dir = std::env::temp_dir().join(format!(
             "tty7-ssh-config-test-{name}-{}",