Incompatible with IntelliJ Rust

[Dinnerbone]

When using IntelliJ Rust, cargo test is specialcased to add extra arguments to allow the IDE to parse the results. Unfortunately, these arguments aren't supported and so you can't run tests from inside the IDE if using libtest-mimic.

When running cargo test it actually adds: --format=json -Z unstable-options --show-output

• --format=json is tracked by #1
• -Z unstable-options Is used to enable --format=json
• --show-output perhaps we should also just do nothing, since that's the current behaviour I believe?

Libtest --help output says:

    --show-output   Show captured stdout of successful tests             
-Z unstable-options Enable nightly-only flags:                           
                    unstable-options = Allow use of experimental features