compiletest: `-Cincremental` in ui tests dump incr build output to source directories

[jieyouxu]

See #128038:

Currently tests can ICE when the test spits out inc-fat incremental artifacts directly into the top of the git checkout, and then the compiler version changes, and it reads nonsense incremental artifacts on a subsequent test run.

I think the right flag to add when porting -Cincremental run-make tests is to use //@ incremental rather than manually specifying the -Cincremental rustflag.

If manual -Cincremental specified in compile-flags cause output to the working directory (not even build directory), then we really should error and suggest //@ incremental.

[compiler-errors]

Either that, or we can set the working directory of the rustc invokation to be in built/$TARGET/test/test_name/. Perhaps that's the better option? IDK what that messes up tho.

[jieyouxu]

Either that, or we can set the working directory of the rustc invokation to be in built/$TARGET/test/test_name/. Perhaps that's the better option? IDK what that messes up tho.

The safest option is probably to do both lol