Fix memory leak in _MatchesReferenceImage

[ksokolovskyi]

This PR fixes a memory leak in the _MatchesReferenceImage, which did not dispose created ui.Image.

Description

• Fixes Memory Leak: _MatchesReferenceImage matcher does not dispose ui.Image #135137.

Tests

• Adds new test to packages/flutter_test/test/reference_image_test.dart to verify that matchesReferenceImage matcher works with widget finders and disposes created ui.Image;
• Updates packages/flutter_test/test/reference_image_test.dart to use testWidgetsWithLeakTracking.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• 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].
• All existing and new tests are passing.

[ksokolovskyi]

cc @polina-c

[polina-c]

One comment.

[ksokolovskyi]

@polina-c thanks for the review!