The test framework should not be autoloaded in production mode.  Before
this commit, the testing railtie would extend AS::TestCase.  This caused
AS::TestCase to be preloaded regardless of the environment in which we
were running.

This commit just moves the code that adds line filtering support in to
the test command where we actually execute the test runner.  That allows
us to maintain the line runner feature but only load the minimal amount
of code we need.