fixed test suite 'WebPage render image'

Previously the test suite 'WebPage render image' made a series of
webpage.open without waiting them to complete. This effected next
runned tests because on load handlers for pages were fired after
'WebPage render image' test finish.

In addition this fix caused tests running time reduction on my
system from 46.559 seconds to 0.498 seconds.

https://github.com/ariya/phantomjs/issues/11780