A revamped test runner.

This Python script serves as a master test runner. It will run each test
script with a fresh instance of PhantomJS. The exit code of each script
determines its success or failure.

The existing tests (run-tests.js) are wrapped as one of the test
scripts. This permits a gradual migration to the new test system.

https://github.com/ariya/phantomjs/issues/12439