patch 9.0.0851: terminal mouse test is still flaky

Problem:    Terminal mouse test is still flaky.
Solution:   Also use WaitForAssert().
diff --git a/src/testdir/test_terminal3.vim b/src/testdir/test_terminal3.vim
index 59fba96..f871187 100644
--- a/src/testdir/test_terminal3.vim
+++ b/src/testdir/test_terminal3.vim
@@ -716,7 +716,7 @@
   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