Syntactically correct way of excluding Rust test code

[arielb1]

As far as I can tell, there is no easy way in grcov to exclude all code that is included in #[cfg(test)] from test coverage. You can use ---excl-start, but there is no way to tell where the #[cfg(test)] block ends.

It would be good if there was some token-tree based mechanism for this.

I can try to implement a PR if this is desired.

[arielb1]

Another possibility would be to have an option that takes a list of lines to ignore per file, which can be computed in a smart way by an external process. Maybe it's smarter to filter the .gcno file.