Add a command-line option --local-urls={true,false}

The default is 'true'.  When set 'false', file: and qrc: URLs are
treated as invalid (unknown scheme) rather than opening local files,
as requested in issue #12752.

In order to test this, I added a mechanism to test/run-tests.py
allowing individual tests to be annotated with command-line
options to pass to phantomjs or the script.