Fixture support (#127)
Fix tests
Major refactoring of test loading code and related tests.

* Get rid of test contexts
* Get rid of older test_generator factility in tests
* Preparation for loading tests using fixtures and fixture variations
Improve tests for slash run and suite verification in unit tests
Fix tests
Integrate fixtures into slash

* Improve fixture namespace tracking and scoping
* Improve suite generator utility in unit tests
Fix cartesian computation of fixtures

Make sure that when we have f1(x, f2), f2(y, x), we get |x| * |y| options, rather than |x|*|x|*|y| options
Refactor test parameters to work better with fixture

* Unify parameters.iterate with slash.parametrize
* Can now be uniformly used for both fixtures and tests
Fix tests
Implement proper fixture scoping and cleanups
Add coverage info via pytest-cov
Improve coverage, remove dead code
Improve coverage
More fixture scoping test cases
Add test for explicit parametrization overrides across test class inheritence
Add tests, verifying that unnecessary fixtures are not initialized
Cosmetics
Add test for fixture initialization order
Fix fixture cleanups for last tests in session
Add utility for iterating cartesian variations as dictionaries