Fix Python completion failure under certain conditions

Fix Python completion failure under certain conditions

* lisp/progmodes/python.el (python-shell-send-string-no-output):
Save and restore `comint-last-prompt-overlay' or `comint-last-prompt'.
* test/lisp/progmodes/python-tests.el (python-tests-shell-wait-for-prompt):
New helper function.
(python-tests-with-temp-buffer-with-shell): New helper macro.
(python-shell-completion-1, python-shell-completion-native-1)
(python-shell-completion-native-with-ffap-1)
(python-shell-completion-native-with-eldoc-1): New tests (bug#58389).