Improve coverage speed by using new caching option for package:coverage (#107395)

Speedup coverage test runs by using (new) coverage handle with caching.

On a `flutter test --coverage` run on `packages/flutter` the runtime goes from ~828 seconds to ~617 seconds, or a ~25% reduction in time spent (testing without coverage takes ~236 seconds so the overhead of `--coverage` in this case goes from ~592 seconds to ~381 seconds, or a ~35% reduction).