gh-109615: Fix support test_copy_python_src_ignore() (#109958)

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".