Manually run x fmt on 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` and
`x test coverage --bless`.

tests/coverage/coroutine.coverage

@@ -18,7 +18,8 @@
    LL|       |
    LL|      1|fn main() {
    LL|      1|    let is_true = std::env::args().len() == 1;
-   LL|      1|    let mut coroutine = #[coroutine] || {
+   LL|      1|    let mut coroutine = #[coroutine]
+   LL|      1|    || {
    LL|      1|        yield get_u32(is_true);
    LL|      1|        return "foo";
    LL|      1|    };