Revert "Ignore includers of untested mixins (#13611)"

[Hexcles]

This reverts commit e9e06cd.

This broke some IDL tests. Some intended subtests were dropped because of this change. Example: https://chromium-review.googlesource.com/c/chromium/src/+/1307069/4/third_party/WebKit/LayoutTests/external/wpt/css/css-transitions/idlharness-expected.txt (all the FAIL lines disappeared, which caused the removal of the whole expectation file.)

[Hexcles]

@foolip looks like an Azure flake:

https://dev.azure.com/web-platform-tests/wpt/_build/results?buildId=136&view=logs

System.InvalidOperationException: Git fetch failed with exit code: 128
   at Agent.Plugins.Repository.GitSourceProvider.GetSourceAsync(AgentTaskPluginExecutionContext executionContext, RepositoryResource repository, CancellationToken cancellationToken)
   at Agent.Plugins.Repository.CheckoutTask.RunAsync(AgentTaskPluginExecutionContext executionContext, CancellationToken token)
   at Agent.PluginHost.Program.Main(String[] args)

I'm admin-merging this PR for now.