Change how runmake v2 tests are executed
This PR makes execution of v2 runmake tests more sane, by executing each test in a temporary directory by default, rather than running it inside `tests/run-make`. This will have.. a lot of conflicts.
Fixes: https://github.com/rust-lang/rust/issues/126080
Closes https://github.com/rust-lang/rust/issues/125726, because it removes `tmp_dir`, lol.
r? `@jieyouxu`
try-job: x86_64-msvc