patch 9.0.0968: GUI mouse event test is a bit flaky

Problem:    GUI mouse event test is a bit flaky.
Solution:   Mark the test case as flaky.  Move test function failure checks to
            a separate test function.

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    968,
 /**/
     967,
 /**/