patch 8.2.5064: no test for what 8.1.0052 fixes

Problem:    No test for what 8.1.0052 fixes.
Solution:   Add a test. (closes #10531)
diff --git a/src/getchar.c b/src/getchar.c
index 03d90ac..2747423 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -3458,8 +3458,8 @@
  * get a character: 3. from the user - get it
  */
 		if (typebuf.tb_len == 0)
-		    // timedout may have been set while waiting for a mapping
-		    // that has a <Nop> RHS.
+		    // timedout may have been set if a mapping with empty RHS
+		    // fully matched while longer mappings timed out.
 		    timedout = FALSE;
 
 		if (advance)