Fix the test when run on an installed Python: use "abs_srcdir" of
sysconfig, and skip the test if the Python source code cannot be
found.
* Tools/patchcheck/patchcheck.py, Tools/freeze/test/freeze.py and
Lib/test/libregrtest/utils.py now first try to get "abs_srcdir"
from sysconfig, before getting "srcdir" from sysconfig.
* test.pythoninfo logs sysconfig "abs_srcdir".
I've also added a small comment to `Tools/c-analyzer/cpython/_parser.py` to trigger the `patchcheck` CI. It must pass now.
Automerge-Triggered-By: GH:ericsnowcurrently