Fix tests due to Firefox function property changes.

Fixes <https://code.google.com/p/google-caja/issues/detail?id=1948>.
All test failures were due to harmless behavior changes in Firefox
that broke assumptions made by the tests.

See <https://code.google.com/p/google-caja/issues/detail?id=1947> for
information on the Firefox changes (function instances no longer have
the caller and arguments properties).

Additionally, increase the timeout for guest-scan test. It takes a
very long time when running on Firefox _under webdriver_. Would like
to fix it, but for now, accept it.

@r5706

[ihab.awad, 2014-12-17 22:47:27]

lgtm