compiler: Running compiler tests in parallel might fail due to shared paths

most compiler tests are reusing `/tmp/foo/` for storage. This all works well if `test_compiler` is called once. But if one calls all subtests in their own processes in parallel to speed up test iterations, some tests might fail randomly as they race each other on directory creation/removal.

Often (only?) seen in tests testing including headers.

Start by locating test_compiler and the compiler tests that use /tmp/foo/, especially the tests involving header inclusion. Run the relevant subtests in separate processes and in parallel to reproduce the race. Done means parallel compiler test runs no longer fail intermittently because tests create or remove shared temporary paths.

Written by the indexing model from the issue text.