run-make: do not run test with the test sources directory as its CWD #126080
Description
The original infra PR setting up rmake.rs version of run-make tests copied over the logic for the legacy Makefile version, which ran the test with the test sources directory as the CWD, which is Very Naughty because if you aren't careful with relative paths, you suddenly can produce output artifacts into your test sources directory. Oops.
We should stop doing that.
cc @Kobzol as they wanted to work on a fix for this.