patch 8.1.1524: tests are silently skipped

Problem:    Tests are silently skipped.
Solution:   Throw an exception for skipped tests in more places.
diff --git a/src/testdir/test_assert.vim b/src/testdir/test_assert.vim
index 7df641c..0d59a43 100644
--- a/src/testdir/test_assert.vim
+++ b/src/testdir/test_assert.vim
@@ -222,6 +222,25 @@
   call assert_fails("call test_override('redraw', 'yes')", 'E474')
 endfunc
 
+func Test_mouse_position()
+  let save_mouse = &mouse
+  set mouse=a
+  new
+  call setline(1, ['line one', 'line two'])
+  call assert_equal([0, 1, 1, 0], getpos('.'))
+  call test_setmouse(1, 5)
+  call feedkeys("\<LeftMouse>", "xt")
+  call assert_equal([0, 1, 5, 0], getpos('.'))
+  call test_setmouse(2, 20)
+  call feedkeys("\<LeftMouse>", "xt")
+  call assert_equal([0, 2, 8, 0], getpos('.'))
+  call test_setmouse(5, 1)
+  call feedkeys("\<LeftMouse>", "xt")
+  call assert_equal([0, 2, 1, 0], getpos('.'))
+  bwipe!
+  let &mouse = save_mouse
+endfunc
+
 func Test_user_is_happy()
   smile
   sleep 300m