Save coverage even if tests fail

[clarfonthey]

It would be nice to have the option to be able to still generate coverage even if tests fail, since it can still be useful even if the code needs to be modified for them to pass. Right now, running cargo llvm-cov --html --open will not generate the coverage and not open it if the tests fail.

[taiki-e]

This should already be available by using two commands, but I'm fine to add a specific option for it.

cargo llvm-cov --no-report --no-fail-fast || true
cargo llvm-cov --no-run --open

[taiki-e]

If we add an option for this (probably --ignore-run-fail?):

• Only run failures should be ignored. i.e., a build failure should fail as usual.
  • This is what commands I mentioned in the above comment do not handle well.
  • To implement this, first run cargo test with --no-run flag, then run regular cargo test, ignoring only the latter error. (related code)
• If this option is specified, the --no-fail-fast flag must be passed to cargo. (Without it, the test will be finished on failure.)
  • To avoid confusion, it is preferable to reject cases where both are used at the same time by using clap's conflicts option.