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.

tests/coverage/long_and_wide.coverage

@@ -1,4 +1,4 @@
-   LL|       |// compile-flags: --edition=2021
+   LL|       |// edition: 2021
    LL|       |// ignore-tidy-linelength
    LL|       |
    LL|       |// This file deliberately contains line and column numbers larger than 127,