Possible bug in simple index test suite

[alanbato]

While working on some related tests for my proof of concept solution to #726, I came across this test
It appears that we're trying to test different combinations of file types and version formats, but not all version formats are being actually used.

https://github.com/pypa/warehouse/blob/fea98d9c054ebae63bd5d1fc478428d2efde6670/tests/unit/legacy/api/test_simple.py#L144-L158

If this is not intended, I'd be happy to submit a small fix :)

[di]

I think that test could probably use some improvement, it's a bit old at this point and could benefit from parametrization, at the very lease. 🙂