patch 8.2.4366: not enough tests for command line completion

Problem:    Not enough tests for command line completion.
Solution:   Add a few more tests. (Yegappan Lakshmanan, closes #9760)
diff --git a/src/cmdexpand.c b/src/cmdexpand.c
index 8e950ab..2a65a06 100644
--- a/src/cmdexpand.c
+++ b/src/cmdexpand.c
@@ -50,6 +50,9 @@
     return STRCMP(p1, p2);
 }
 
+/*
+ * Escape special characters in the cmdline completion matches.
+ */
     static void
 ExpandEscape(
     expand_T	*xp,