patch 9.0.2102: matchparen highlight not cleared in completion mode

patch 9.0.2102: matchparen highlight not cleared in completion mode

Problem:  matchparen highlight not cleared in completion mode
Solution: Clear matchparen highlighting in completion mode

Remove hard-coded hack in insexpand.c to clear the :3match before
displaying the completion menu.

Add a test for matchparen highlighting. While at it, move all test tests
related to the matchparen plugin into a separate test file.

closes: #13493
closes: #13524

Signed-off-by: Christian Brabandt <cb@256bit.org>