Default to using test-unit gem even if minitest_tu_shim is installed

[joast]

Force test-unit gem to be used unless "USE_TEST_UNIT=no" is set in the environment. If "USE_TEST_UNIT=no" is set in the environment and minitest_tu_shim is installed, then minitest will be used instead of test-unit. If minitest_tu_shim isn't installed, then the environment variable doesn't matter and test-unit will be used.