Do more tests on polynomial multiplication

[teor2345]

In #9 we're going to modify the polynomial multiplication algorithm.

But we don't have comprehensive tests yet, we're missing some edge cases like:

• [ ] multiplication by 0
• [ ] multiplication by 1
• [ ] multiplication by -1
• [ ] multiplication by X
• [ ] TODO: any more edge cases?

And we don't have full coverage of our implementation:

• [ ] test the two polynomial modulus implementations with all the existing test cases
• [x] test the three polynomial multiplication implementations with all the existing test cases (done in #32)

We might also want to:

• [ ] bias the degree of the polynomial towards edge cases like X^0, X^1, X^{N-1}, X^N (partly done for some tests)
• [ ] bias the coefficients of the polynomial towards edge cases like 0, 1, MODULUS-1

Let's implement a reasonable number of edge cases, then close this ticket. We don't need to do them all.

[teor2345]

We’re pretty confident this works, so I don’t think we need extra tests.