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.

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

This is a perfect example of LBYL going wrong: that code could be executed
by several workers in parallel, and os.mkdir() attempted on the same
path by multiple processes.

coverage.py reports 99% on branch coverage, but that appears to be
a bug or limitation in coverage.py.