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