bug fixes

- make run_one_set_of_tests return a TestResult
- add the -P/--path flag to enable adding additional libs to sys.path

With these changes it should be possible to run mojo_python_tests
and webkit_python_tests properly from the command line.

I still need to come up with a better way of using the stock TestResult
object to track overall status while still tracking stdout and stderr
for each test and the results of each successful test.