Fix test-tooltip.lua: Run the event loop

This test changes the mouse cursor's position and afterwards has an
assert that checks something on the tooltip. This really looks a lot
like it expects the mouse cursor's position to be already updated and
its enter and leave events to be handled. However, this is now how
things actually work.

Fix this by moving the assert into its own step, so that in between the
normal main loop runs.

Signed-off-by: Uli Schlachter <psychon@znc.in>

tests/test-tooltip.lua

@@ -93,8 +93,12 @@ table.insert(steps, function()
         x = w.x + w.width - 20 - 12.5,
         y = w.y + 125 + 12.5,
     }
+    return true
+end)
 
-    assert(tt.current_position == "top")
+-- Test that the above move had the intended effect
+table.insert(steps, function()
+    assert(tt.current_position == "top", tt.current_position)
 
     return true
 end)