utf8 normalizer tests break test suite when network connection is not available

Feel free to downgrade priority if you think major is too high.

It seems that setUpBeforeClass() is called regardless of whether or not the tests are in a skipped group. As a result, if there is no internet connection available, utf8 normalizer tests try to fetch the files regardless and throw an exception which destroys the entire test run.

There are multiple problems here:

1. Obviously, setting up a test case is pointless if it is not going to run.
2. Uncaught exceptions in tests should not destroy the run. They should be caught by the test framework.

The "fix" is to use trigger_error instead of throwing exceptions.