patch 8.2.4228: no tests for clicking in the GUI tabline

Problem:    No tests for clicking in the GUI tabline.
Solution:   Add test functions to generate the events.  Add tests using the
            functions. (Yegappan Lakshmanan, closes #9638)
diff --git a/src/normal.c b/src/normal.c
index b6c59d3..e69c141 100644
--- a/src/normal.c
+++ b/src/normal.c
@@ -2801,8 +2801,9 @@
 }
 
 /*
- * Get the count specified after a 'z' command. Returns TRUE to process
- * the 'z' command and FALSE to skip it.
+ * Get the count specified after a 'z' command. Only the 'z<CR>', 'zl', 'zh',
+ * 'z<Left>', and 'z<Right>' commands accept a count after 'z'.
+ * Returns TRUE to process the 'z' command and FALSE to skip it.
  */
     static int
 nv_z_get_count(cmdarg_T *cap, int *nchar_arg)