Change tests to refer to the Performance Platform.

Now that we don't have tests for the Backdrop and Limelight
applications (since commit 84afb4f4f9d1), it makes more sense to most
people on the GOV.UK team to refer to the remaining tests as testing the
Performance Platform as a whole.
Merge pull request #115 from alphagov/performance-platform-tests

Change tests to refer to the Performance Platform