Add documentation for Java test filtering to plugins test document (#165314)

Picked up from flutter/flutter#155761 
These docs look helpful, picking up to get them in!

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

Author: Piinks

docs/ecosystem/testing/Plugin-Tests.md

@@ -91,6 +91,16 @@ once the example app is opened as an Xcode project.
 
 For Windows plugins, Visual Studio should auto-detect the tests and allow running them as usual.
 
+#### Filtering Android native tests
+
+The Packages repo's Flutter tool calls Gradle commands in order to run native tests. After running `dart run flutter_plugin_tools.dart native-test`, the output log will contain a line noting the command run, such as:
+
+    /path/to/gradlew app:testDebugUnitTest package_name_here:testDebugUnitTest
+
+Running this command manually with the `--tests` flag allows [test filtering](https://docs.gradle.org/current/userguide/java_testing.html#test_filtering). For example, the following command will run only the tests in `ConvertTest.java` for `google_maps_flutter_android`:
+
+    /path/to/gradlew app:testDebugUnitTest google_maps_flutter_android:testDebugUnitTest --tests "io.flutter.plugins.googlemaps.ConvertTest"
+
 ### Web Tests
 
 Most web tests are written as Integration Tests because they need a web browser (like Chrome) to run. Web integration tests are located in the `example` directory of the `<plugin_name_web>` package.