tests: test_pgdata_import_smoke requires the 'testing' cargo feature (#10569)

It took me ages to figure out why it was failing on my laptop. What I
saw was that when the test makes the 'import_pgdata' in the pageserver,
the pageserver actually performs a regular 'bootstrap' timeline creation
by running initdb, with no importing. It boiled down to the json request
that the test uses:

```
        {
            "new_timeline_id": str(timeline_id),
            "import_pgdata": {
                "idempotency_key": str(idempotency),
                "location": {"LocalFs": {"path": str(importbucket.absolute())}},
            },
        },
```

and how serde deserializes into rust structs. The 'LocalFs' enum variant
in `models.rs` is gated on the 'testing' cargo feature. On a non-testing
build, that got deserialized into the default Bootstrap enum variant, as
a valid TimelineCreateRequestModeImportPgdata variant could not be
formed.

PS. IMHO we should get rid of the testing feature, compile in all the
functionality, and have a runtime flag to disable anything dangeorous.
With that, you would've gotten a nice "feature only enabled in testing
mode" error in this case, or the test would've simply worked. But that's
another story.

test_runner/regress/test_import_pgdata.py

@@ -59,6 +59,9 @@ def test_pgdata_import_smoke(
     neon_env_builder.enable_pageserver_remote_storage(RemoteStorageKind.LOCAL_FS)
     env = neon_env_builder.init_start()
 
+    # The test needs LocalFs support, which is only built in testing mode.
+    env.pageserver.is_testing_enabled_or_skip()
+
     env.pageserver.patch_config_toml_nonrecursive(
         {
             "import_pgdata_upcall_api": f"http://{cplane_mgmt_api_server.host}:{cplane_mgmt_api_server.port}/path/to/mgmt/api"