LTI external tests clean up

There are a few issues here which should be fixed:

1. A number of the tests for external.php in mod/lti/tests/externallib_test.php don't clean the return value before making assertions about the return. We should always call clean_returnvalue().

2. The test exploits the setUp method to create courses and enrolments which are then only used in a couple of tests. This should be achieved via a helper method instead.

3. A number of tests check for exceptional behaviour using a try/catch in the test. We should replace this with expectException() calls.

4. There are a number of unused vars.