patch 8.1.1321: no docs or tests for listener functions

Problem:    No docs or tests for listener functions.
Solution:   Add help and tests for listener_add() and listener_remove().
            Invoke the callbacks before redrawing.
diff --git a/src/testdir/Make_all.mak b/src/testdir/Make_all.mak
index 1f50bd8..7dc7e36 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -168,6 +168,7 @@
 	test_lispwords \
 	test_listchars \
 	test_listdict \
+	test_listener \
 	test_listlbr \
 	test_listlbr_utf8 \
 	test_lua \
@@ -359,6 +360,7 @@
 	test_lineending.res \
 	test_listchars.res \
 	test_listdict.res \
+	test_listener.res \
 	test_listlbr.res \
 	test_lua.res \
 	test_makeencoding.res \