When making changes that have a large impact on coverage counter creation, this
makes it easier to see whether the number of physical counters has changed.
(The highest counter ID seen in coverage maps is not necessarily the same as
the number of physical counters actually used by the instrumented code, but
it's the best approximation we can get from looking only at the coverage maps,
and it should be reasonably accurate in most cases.)
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`.
