The --start option to regrtest raises an AttributeError in many scenarios.  For example, these both raise an error:

$ ./python.exe -m test --start test_random
$ ./python.exe -m test --start test_random test_binascii test_random

Traceback (most recent call last):
  File ".../Lib/runpy.py", line 160, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File ".../Lib/runpy.py", line 73, in _run_code
    exec(code, run_globals)
  File ".../Lib/test/__main__.py", line 13, in <module>
    regrtest.main()
  File ".../Lib/test/regrtest.py", line 562, in main
    del tests[:tests.index(start)]
AttributeError: 'NoneType' object has no attribute 'index'

Patch attached.