Partial work towards https://github.com/flutter/flutter/issues/148420, unblocks https://github.com/flutter/engine/pull/55537. These tests can now be more precise, and changing the fixtures no longer has side-effects on tests across the entire repository. There are about 11 other usages (there were ~50 in these) after this PR that I'll get to, as well as the hard-coded `gn desc` output, before retiring `fixtures.dart`