Explain how to execute a single UI test

[phansch]

This adds a section to Running test suite that explains how to run a single test file.

On my old machine it took a few minutes for the whole suite to finish and running a single test is useful to have a shorter feedback loop, even on a faster machine.

[phansch]

I'm just not sure if this section should mention that the command doesn't work on files that depend on other crates. For example TESTNAME=ui/regex cargo test --test compile-test would fail.

[bootandy]

This is a good idea. I was wondering how to do that myself. (I was just deleting all the other ui tests in the directory)

[oli-obk]

I'm just not sure if this section should mention that the command doesn't work on files that depend on other crates.

Curious. I wonder if anyone ever noticed that in the compiler test suite :D

[phansch]

Wait, with the latest Rust release, incremental compilation is now on by default right? There is also rust-lang/cargo#4817 which enabled incremental compilation in cargo by default, so I think CARGO_INCREMENTAL=1 is not needed anymore. I will test it later at home though, and make a PR to remove that part again :)