Make comint-tests more robust (bug#38813)

Make comint-tests more robust (bug#38813)

* test/lisp/comint-tests.el (comint-test-no-password-function)
(comint-test-password-function-with-value)
(comint-test-password-function-with-nil):
Call accept-process-output as many times as needed, with a slightly
more generous timeout (100 ms), after sending the Password: prompt to
the process, since there must be time for some back-and-forth
communication.  Also clear the process-query-on-exit flag, since it
doesn't go well with noninteractive tests.