fix(tests): Fix test failures by reducing shared global state.

* Completely clear Session, localStorage, and sessionStorage before and after every test.
* In the FxaClientWrapper, add `testClear` to clear global state.
* Minimize possibility of FxaClientWrapper collisions by using wrapper that is created by the views.

fixes #863
fix(tests): Use `assert(false, <message>)` instead of `assert.fail(<message>)`.

* The latter is incorrect usage according to the Chai docs.

fixes #861
chore(tests): Remove the unneeded fxa-client.js dependency.
Merge pull request #864 from mozilla/issue-863-test-failure

Issue 863 test failure

A slight break in protocol, I'm self merging. @nchapman did a review and said things are looking good. After > 5 green runs on travis, I'm going to say this is good enough to get us moving again. Will follow on with functional test changes in a separate PR.