Fixed leak and removed no-shuffle tag in image_stream_test.dart - #88376

Fixes test/painting/image_stream_test.dart of #85160

The problem

The timeDilation was changed to 2.0 but not restored. The changed timeDilation carried on to following tests which messed up some of them when using test ordering seed 456.

Fix

Restore timeDilation to 1.0 at the end of test.

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 feature I am adding, or Hixie said the PR is test-exempt.
• All existing and new tests are passing.