[native assets] `dart test` with pub workspaces

[dcharkes]

https://github.com/dart-lang/native/actions/runs/14316483742/job/40123703746?pr=2178

Run dart --enable-experiment=native-assets test

❌ test/add_test.dart: invoke native function (failed)
  Invalid argument(s): Couldn't resolve native function 'add' in 'package:native_dynamic_linking_example/add.dart' : No asset with id 'package:native_dynamic_linking_example/add.dart' found. No available native assets. Attempted to fallback to process lookup. dlsym(RTLD_DEFAULT, add): symbol not found.
  
  dart:ffi                                         Native._ffi_resolver_function
  package:native_dynamic_linking_example/add.dart  add
  test/add_test.dart [10](https://github.com/dart-lang/native/actions/runs/14316483742/job/40123703746?pr=2178#step:12:11):12                         main.<fn>

The hook is being invoked and the assets are being built.

No available native assets.

However, the native_assets.yaml is not being used.

[dcharkes]

package:test expects the native_assets.yaml to be next to the package_config.json (dart-lang/test#1975).

However, with pub workspaces, the native_assets.yaml is in the .dart_tool of the root package, rather than in the .dart_tool of the workspace.

[dcharkes]

Fixed in two places:

• 423479c
• [native assets] Add support for pub workspaces test#2484