lint on a call to `exit` in tests

[oli-obk]

cc rust-lang/rust#33954

Should be easy-enough once @llogiq's cross crate analyzer is up and running.

[llogiq]

@Manishearth We should have a T-Test-related label.

[Manishearth]

Well, T- is for categorizing how the lint is to be written, and is basically used to make the E- labels more finer grained.

But +1 for A-Test and A-style and whatnot. Or some other prefix (servo uses A-)

[llogiq]

Fine with me.

[felix91gr]

Is this still a potential lint? I'd like to give it a try! :)

[oli-obk]

While still a potential lint, Without cross crate analysis capabilities or MIR-only-rlibs, any implementation we could do is pretty restricted

[felix91gr]

Hmm. I see. Are those things coming? If not, maybe this can be tagged or otherwise noted somewhere as "good idea but necessary tooling isn't coming up anytime soon"?