run.py: Implement tests

[jugglinmike]

The run.py currently has minimal testing coverage. To promote safety during
feature addition and refactoring (e.g. gh-426), test coverage should be
improved.

(Because we are not yet sure of the way we will distribute concurrent test
execution, I will be writing these tests in a high-level integration style,
which stub out the wpt binary provided by the Web Platform Tests project.
This should help ensure the tests stay relevant across future refactorings.)

[mariestaver]

Functional tests are added -- more will be added as needed -- closing issue