Rollup merge of #145766 - epage:rustfmt, r=calebcartwright

test(rustfmt): Verify frontmatter is preserved

This is to prove that the frontmatter is preserved.

The choices in tests is intended for showing the different parts of the proposed Style Guide for frontmatters (rust-lang/rust#145617).

While rustfmt is developed in a different repo, work involving upstream integration is blocked on some work that is being finished up in that repo.  I was told that it would be ok to post against this repo in the mean time.

Tracking issue: rust-lang/rust#136889

Author: Zalathar