gh-109972: Split test_gdb.py into test_gdb package (#109977)

Split test_gdb.py file into a test_gdb package made of multiple
tests, so tests can now be run in parallel.

* Create Lib/test/test_gdb/ directory.
* Split test_gdb.py into multiple files in Lib/test/test_gdb/
  directory.
* Move Lib/test/gdb_sample.py to Lib/test/test_gdb/ directory.
  Update get_sample_script(): use __file__ to locate gdb_sample.py.
* Move gdb_has_frame_select() and HAS_PYUP_PYDOWN to test_misc.py.
* Explicitly skip test_gdb on Windows. Previously, test_gdb was
  skipped even if gdb was available because of
  gdb_has_frame_select().