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.

The Fix: Restore timeDilation to 1.0 at the end of test.