Oh, for test_regrtest which hangs on Windows, I'm quite sure that it comes from test_crashed() which opens a popup!
In the master branch, test_crashed() uses faulthandler._sigsegv() which disables the Windows popup using SetErrorMode() and _set_abort_behavior().
In Python 2.7, test_crashed() just calls ctypes.string_at(0) which doesn't prevent the Windows popup.
My PR https://github.com/python/cpython/pull/2423 implements support.SuppressCrashReport on Windows and so will allow to fix this issue!