Move webapi/tests.py into webapi/tests/.

This is the first change in a series to split up the webapi unit test
suite. It just moves the existing tests into a new webapi/tests/
subdirectory, and updates testdata file paths to work from the new
location.

No logic has changed, and tests can still be executed as before.
However, you may need to delete your webapi/tests.pyc file.