V4 mocha middleware

First pass at removing the need for a separate `assemble:tests` task

TODO:
- [x] Load the IE Sinon helper based on UA string
- [x] Remove Assemble test task
- [x] Change placement of Mocha div
- [x] Cleanup commits
- [x] Fix failing init tests unless @patheard says he will PR afterwards

/cc @patheard