Move unit tests to separate directory

Move unit tests to tests/unit directory for each component. This is in preparation for adding integration tests. Modify test path in Makefile and tox.ini.
Move fixtures and resource back to root of tests

Unit and integration may share fixtures and other resources. Move those directories from the unit directory back to the root of the tests directory. Refactor tests on where to look for these resources.
Merge pull request #911 from StackStorm/STORM-883/mistral-integration-tests

Move unit tests to separate directory