more specific tests for 'wait' parameter

See https://github.com/scrapinghub/splash/pull/22 for the discussion.

Other idea from #22 is to limit 'wait' to 10s. I haven't implemented it here (wait is still limited to 60s) - for example, there could be pages with decreasing counter that allows to do action after some time, and this time can be >10s. I don't know if it a common enough scenario. What do you think?