Fix the intermittent failing front end tests.

The session tests attach a test "channel". "channel" cannot be cleared, and since Session is a singleton, the test channel never goes away. in fxa-clien.js->signIn, we attempt to send a message to a channel without a send function.

fixes #516
Merge pull request #517 from mozilla/issue-516-failing-tests

Fix the intermittent failing front end tests.