test-bundle2-format: force gc so a GeneratorExit will be thrown
PyPy has looser semantics than CPython for when a generator's close
method will be called. Forcing the gc causes it to be called at
the right moment.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.