Auto merge of #126097 - Kobzol:runmake-change-cwd, r=jieyouxu

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: rust-lang/rust#126080
Closes rust-lang/rust#125726, because it removes `tmp_dir`, lol.

r? `@jieyouxu`

try-job: x86_64-msvc

Author: bors