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` and
`x test coverage --bless`.
For coverage tests, splitting code across multiple lines often makes the
resulting coverage report easier to interpret, so we force rustfmt to retain
line breaks by adding dummy line comments with `//`.
