coverage: Use normal `edition:` headers in coverage tests

[Zalathar]

Some of these tests were originally written as part of a custom run-make test, so at that time they weren't able to use the normal compiletest header directive parser.

Now that they're properly integrated, there's no need for them to use compile-flags to specify the edition, since they can use edition instead.

In most cases the .cov-map snapshot isn't affected at all, but in a few cases we add or remove a line, which slightly disturbs the first line number in each instrumented function.

[rustbot]

r? @Mark-Simulacrum

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[compiler-errors]

r? compiler-errors @bors r+ rollup

[bors]

:pushpin: Commit c9c049b2b42f4c05ec49fd1c9125fa4333badd3f has been approved by compiler-errors

It is now in the queue for this repository.

[bors]

:evergreen_tree: The tree is currently closed for pull requests below priority 100. This pull request will be tested once the tree is reopened.