Format all source files in `tests/coverage/`
Currently we can't automatically enforce formatting on tests (see #125637), but we can at least keep things relatively tidy by occasionally running the formatter manually.
This was done by temporarily commenting out the `"/tests/"` exclusion in `rustfmt.toml`, and then running:
- `x fmt tests/coverage`
- `x test coverage --bless`
(This PR also includes a few cosmetic tweaks to some of the affected files, to convince rustfmt to format them in the way we want.)
``@rustbot`` label +A-code-coverage
a7a2fa51f1