Navigation fixtures no longer take args

- This allows for reusing an open browser in the legacy tests
- They also work the same in and out of py.test.
- Fortunately, there wasn't a lot of nav fixtures being called as functions,
  so it was a tiny refactor
- Also removed the NAvigatoinFixture class, as it was only doing functional work
Merge pull request #293 from seandst/add-provider-fixes

Navigation fixtures no longer take args