Fix formatter to correctly convert invalid doc comments

[haydenbaker]

Background

• Fixes an issue where a doc-comment above a trait-statement and member may get converted to a line-comment
• Updates the expected formatting of test case file (.formatted.smithy)
• Updates tests to use assertEquals, since it provides diff functionality in IDEs

Testing

• Ran the tests, no failures

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[milesziemer]

I pushed a commit that fixes the regression, which is captured by what I added to the test case here. The TODO here is a separate issue that occurs when the comment is after a default value assignment. We will need to follow up to address that.

[milesziemer]

Followup work: #2279