Physically remove the folder src/coreclr/tests (#42828)

Physically remove the folder src/coreclr/tests (#42828)

This change removes the last few files from src/coreclr/tests
under src/tests and finally depletes the folder. One change of note
is rename of src/coreclr/tests/runtest.py to src/tests/run.py
in accordance with the previous renames.

I have put the runparallel.sh script to src/tests as it seems
to have general usability, I moved the rest of the scripts to
src/tests/Common/scripts. I haven't actually tried to make it
work. I managed to catch one reference from arm32_ci_script /
arm_ci_test.sh to bringup_runtest.sh that I fixed but again
without actually testing them.

I am not too familiar with these scripts, I haven't found
references to them anywhere in the infra, some of them seem
related to the JIT team, adding Bruce to the PR to confirm.
I have no idea whether they are used at all, any insights into
what obsolete scripts we can get rid of is more than welcome.

Thanks

Tomas