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().
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