Refactored test suite into each submodule
This PR refactors the test suite mainly out of gwpy.tests such that the following is true
- tests for a given submodule (e.g.
gwpy.time) live under that sub-module directory (/gwpy/time/tests/) -
gwpy.testsnow just serves some common test infrastructure
This should have no impact on the actual tests run, or the coverage result.