Eventually, all the CPython tests should pass under byterun. You can add CPython tests until some of them fail, and then you've found a bug!
Byterun uses tox to run the test suite under both Python 2.7 and Python 3.3. Tox will create a virtualenv for each Python verison. Here are some useful tox + nosetests commands.
General construction
tox [args to tox] -- [args to nosetests]
Running only one version of Python with tox
tox -e py27 or tox -e py33
Running one test with tox & nosetests
tox -- tests.test_file:TestClass.test_name
Pass the -s flag to make nosetests not capture stdout. Note that because of the byterun tests' structure, this will make the test fail even if it would otherwise pass.
tox -e py33 -- -s tests.test_file:TestClass.test_name