cargo test: allow to specify tests by filename and line

[da-x]

If it was possible to specify tests to run by filename and line of the test function, then editors would not need to work hard in order to invoke specific tests from within the editor. Currently reliance on heuristics or RLS features is needed for this.

[dwijnand]

I think this might need to be opened against the rust-lang/rust repo, seeing as cargo just delegates to its libtest behaviour for test section (such as rust-lang/rust#30422).

[alexcrichton]

Indeed this'd need to be a feature request of libtest in upstream rust-lang/rust, so I'm going to close in favor of an issue there