Migrate off a global test fixture for build and lint. (#55540)

Partial work towards flutter/flutter#148420, unblocks #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`

Author: matanlurey