This gives us a consistent test framework across all Ruby versions which
should help avoid errors that are only found when CI runs the tests on
different Rubies. (And this fixes an immediate bug where there's no
`skip` method in the version of test-unit we're currently using only on
Ruby 2.2.)
If any submodules are missing from grammars.yml, or are listed in
grammars.yml but missing from the repo, the test will fail.
Eventually it would be good to test that the scopes for each submodule
are accurate, but that will take some more work.
It must be sorted, because it will get re-sorted when
script/download-grammars is next run and that would clutter up diffs.
And it must not contain any duplicate scopes.
