Cleanup

Fall cleaning of the test suite is occurring now :) This pull request cleans up several xfails, xpasses, as well as updated several testcases making them more relevant (up to and including deletion). This will get us green builds on stage and prod, it's important to remember when reviewing this pull request that Web QA no longer supports the dev environment.

In particular:
* <code>test_that_plugin_filters_result</code>[[0](https://github.com/m8ttyB/Socorro-Tests/commit/c42b1928ce9b316e3628af79605f244e85bf52eb)] - has been updated to use a different version of Firefox ([bug 913126](https://bugzilla.mozilla.org/show_bug.cgi?id=913126))
* <code>test_that_filter_for_browser_results</code>[[1](https://github.com/m8ttyB/Socorro-Tests/commit/c56eb78711b0be787f6d266cd8efb07a98d15e72)] - removed; no longer relevant

[0] - https://github.com/m8ttyB/Socorro-Tests/commit/c42b1928ce9b316e3628af79605f244e85bf52eb
[1] - https://github.com/m8ttyB/Socorro-Tests/commit/c56eb78711b0be787f6d266cd8efb07a98d15e72