`--cfg coverage` and `--cfg coverage_nightly` not passed on by trybuild

[0b10011]

--cfg coverage and --cfg coverage_nightly get dropped by trybuild, so #[cfg_attr(coverage_nightly, coverage(off))] doesn't get triggered. That results in affected code showing up as being uncovered instead of being ignored.

Support for basic coverage was hackily added:

https://github.com/dtolnay/trybuild/blob/8c1519b64238bd141860f027faa018e966f5517e/src/rustflags.rs#L13-L19

Better support for RUSTFLAGS being passed along is tracked in dtolnay/trybuild#283.