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/sort_groups.coverage

@@ -1,4 +1,4 @@
-   LL|       |// compile-flags: --edition=2021
+   LL|       |// edition: 2021
    LL|       |
    LL|       |// Demonstrate that `sort_subviews.py` can sort instantiation groups into a
    LL|       |// predictable order, while preserving their heterogeneous contents.