gh-109972: Enhance test_gdb (#110026)

* Split test_pycfunction.py: add test_cfunction_full.py.
  Split the function into the following 6 functions. In verbose
  mode, these "pycfunction" tests now log each tested call.

  * test_pycfunction_noargs()
  * test_pycfunction_o()
  * test_pycfunction_varargs()
  * test_pycfunction_varargs_keywords()
  * test_pycfunction_fastcall()
  * test_pycfunction_fastcall_keywords()

* Move get_gdb_repr() to PrettyPrintTests.
* Replace DebuggerTests.get_sample_script() with SAMPLE_SCRIPT.
* Rename checkout_hook_path to CHECKOUT_HOOK_PATH.
* Rename gdb_version to GDB_VERSION_TEXT.
* Replace (gdb_major_version, gdb_minor_version) with GDB_VERSION.
* run_gdb() uses "backslashreplace" error handler instead of "replace".
* Add check_gdb() function to util.py.
* Enhance support.check_cflags_pgo(): check also for sysconfig
  PGO_PROF_USE_FLAG (if available) in compiler flags.
* Move some SkipTest checks to test_gdb/__init__.py.
* Elaborate why gdb cannot be tested on Windows: gdb doesn't support
  PDB debug symbol files.