Merge selenium/tests/behave with berkeleybop/behave_core, clean test directory structure

[kltm]

Merge selenium unit tests with berkeleybop/behave_core. This will involve adding a in importer steps file and getting behave_core into PYTHONPATH (see behave_core tests/ for example).

This will help avoid code duplication.

As long as we're in there, remove the no-longer-useful structures of the directories--there should only be tests/behave remaining, with non-behave stuff in tests/.