Skip tests requiring p4 and svn in test_assembla.py
Skip tests requiring p4 and svn in test_assembla.py

Certain tests in test_assembla.py which involve importing P4 or svn integration
fail if those modules aren't installed. We should skip them instead. Now we
respond to an ImportError being raised by skipping that test.

Testing Done:
Ran unit tests in test_assembla.py - four tests that previously failed due to
ImportError are now skipped.

Reviewed at https://reviews.reviewboard.org/r/7867/

sgallagher/reviewboard,davidt/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,davidt/reviewboard,chipx86/reviewboard,brennie/reviewboard,chipx86/reviewboard,brennie/reviewboard,brennie/reviewboard,chipx86/reviewboard,brennie/reviewboard,sgallagher/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,davidt/reviewboard,davidt/reviewboard,sgallagher/reviewboard,sgallagher/reviewboard