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>
diff --git a/src/testdir/Make_all.mak b/src/testdir/Make_all.mak
index 6b4d8df..1614cf0 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -206,6 +206,7 @@
 	test_matchadd_conceal \
 	test_matchadd_conceal_utf8 \
 	test_matchfuzzy \
+	test_matchparen \
 	test_memory_usage \
 	test_menu \
 	test_messages \