patch 9.0.0851: terminal mouse test is still flaky

Problem:    Terminal mouse test is still flaky.
Solution:   Also use WaitForAssert().

src/testdir/test_terminal3.vim

@@ -716,7 +716,7 @@ func Test_term_mouse()
   call TermWait(buf, 50)
   call term_sendkeys(buf, ":call writefile([@\"], 'Xbuf')\<CR>")
   call WaitFor(XbufExists)
-  call assert_equal('yellow', readfile('Xbuf')[0])
+  call WaitForAssert({-> assert_equal('yellow', readfile('Xbuf')[0])})
   call delete('Xbuf')
 
   " Test for selecting text using double click

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    851,
 /**/
     850,
 /**/