address some comments by @hpk42 on 0b9d82e :
- move tests into their own class, rename
- add test showing metafunc.parametrize called in pytest_generate_tests rather than as decorator
- add test and fix single-argname case
- convert two loops into one in parametrize()
also
- renamed 'input' to 'n', since 'input' is a built-in