added tests of inherit parent page settings

I wrote some tests of https://github.com/ariya/phantomjs/pull/11887.
the master branch build were passed to these tests.
( but this problem is not only a UserAgent setting. we should write about all page.settings.)

could you try to this patch?
and please re-comment to https://github.com/ariya/phantomjs/pull/11887 if you merged.

Regards.