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.
diff --git a/src/version.c b/src/version.c
index 395bb2a..f26f91a 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    968,
+/**/
     967,
 /**/
     966,