Move /tests handler into its own middleware

The /tests behavior was coupled to the auto and location behavior,
despite the fact that all location types will need to map `/tests` to
the `tests/index.html` file.

I've moved tests into its own middleware which also clarifies what the
behavior for /tests is. This refactor corrects a bug where hash location
servers would redirect to `/tests/`.
Merge pull request #1712 from mixonic/tests-without-auto-or-history

Move /tests handler into its own middleware