[OpenMP] Fix libarcher tests on Ubuntu 22.04 (#170671)
When llvm-symbolizer is not found on PATH TSan uses system's addr2line
instead. On Ubuntu 22.04 addr2line can't handle DWARF v5, which results
in failures in some libarcher tests.
This PR adds the directory of the just built LLVM binaries to PATH, to
make llvm-symbolizer available to TSan.
The changes were tested on an AArch64 machine, on which
task-taskgroup-unrelated.c was flaky. Moving the test code to a separate
function, executed 10 times, solved the issue.
Fixes #170138
