Add a common code path for config files used in tests

[TzviPM]

Many tests have code like the following that's used to generate a config file from a yaml string: https://github.com/dart-lang/ffigen/blob/master/test/collision_tests/decl_type_name_collision_test.dart#L21-L30.

Funnelling these through a single code path would make it easier to experiment with changes to the config spec, as codemods could be applied in a single place during testing.

Also, Google's internal use of ffigen requires changes to the file paths for the header files used in tests, and this would decrease the maintenance burden of those changes