[Feature] Live unit testing

[Lokathor]

One thing that would be helpful, is to have an indicator along the status bar which shows just a count of passed tests out of total tests, eg cargo-test: 12/12, or something like that. Green if you've got 100% passed and red if there were failures.

Then RA can just rerun all tests on every save and re-display the value. Lets you keep track as soon you poked something out of alignment.

Clicking the indicator could perhaps even open up a terminal tab with the last set of cargo-test output so that you can check on any error messages.

[matklad]

closing in favor of #8420