Remove the remaining test suite infrastructure.

With the removal of the webrunner there isn't much point in keeping all
the test harness infrastructure in place. TestShell also serves little
purpose now, as a tiny wrapper around phpunit is not super useful. If
there is strife around this change we can always reconsider and come to
a better compromise.
Update PluginTask to generate phpunit boilerplate.

There were a few mistakes in the PluginTask. Also make it easier to get
plugins up and running with PHPUnit by generating some boilerplate code.
Remove duplicate comment.
Merge pull request #2993 from markstory/3.0-testsuite-cleanup

3.0 testsuite cleanup