Add crystal tool format tests

[straight-shoota]

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[ysbaddaden]

I'm not sure all projects use the formatter. I mean that mine usually don't use it, so just checking for the return type may not be enough. For example ysbaddaden/minitest may only pass the formatter check by sheer luck 😅

[straight-shoota]

crystal_format is an explicit test, so we can easily drop it on any project which doesn't maintain the format. We can do that as we notice breakages.

[straight-shoota]

With the formatter changes in the current nightly compiler I realized that formatter tests should not be mixed with functional tests. So I've extracted them into independent test cases which we can filter by tag. Now there's a separate workflow job that tests the formatter exclusively, while all other jobs ignore the formatter checks.

Formatter tests are currently failing, but that's expected because not all shards have adopted the formatter changes in nightly. We should hopefully see a green spec when testing against 1.14, but we can only do that once this PR is merged (automatic builds go against nightly, we can trigger a workflow_dispatch against other builds manually).