MERGE: startswith and endswith don't accept None as slice index. Patch by Torsten Becker. (closes #11828)

Wait for the child process to terminate before ending the test, so that the
regrtest cleanup code doesn't get an error when it tries to delete the
temporary CWD.

Py_AddPendingCall() for the first signal to fix a deadlock on reentrant or
parallel calls. PyErr_SetInterrupt() writes also into the wake up file.