Improve Python triple-quote pairing tests for electric-pair-mode

Improve Python triple-quote pairing tests for electric-pair-mode

These tests were once passing incorrectly, i.e. the auto-pairing
functionality they purport to guard wasn't really working.  Added a
new test in hopes that regressions can be spotted in the future for
the now-working functionality of Python triple-quote auto-pairing via
electric-pair-mode.

bug#49518

* test/lisp/progmodes/python-tests.el
(python-triple-double-quote-pairing): Rename from
python-triple-quote-pairing.
(python-triple-single-quote-pairing): New test.