fix check-license to properly check snippets

[QuietMisdreavus]

Bug/issue #, if applicable: None

Summary

When snippets were introduced to swift-markdown, the check-source script wasn't updated to handle the way the license statement was written. This caused the bin/test script to start failing. This PR updates the check-source script to handle snippet files separately, since they have their license text written at the end of the file, instead of the beginning.

Dependencies

None

Testing

Ensure that bin/test (specifically bin/check-source) succeeds with the snippets in the repo.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• [ n/a ] Added tests
• Ran the ./bin/test script and it succeeded
• [ n/a ] Updated documentation if necessary

[QuietMisdreavus]

@swift-ci Please test

[bitjammer]

LG, thank you!