Improve performance of the polynomial multiplication property test

[TurtlePU]

Currently, one of the slowest tests in our test suite is the test of correctness of FFT polynomial multiplication:

propMultiplication (p1, p2) = p1 * p2 == p1 `naive` p2

This runs slowly because the naive part runs in $O(|p_1| |p_2|)$. I suggest changing it to:

propMultiplication p q x = evalPoly (p * q) x == evalPoly p x * evalPoly q x

Pros:

1. The property is more natural: it checks that multiplication of polynomials works like a pointwise multiplication of evaluation functions;
2. Testing this property is $O(n\log n)$, which is optimal.

Cons:

1. Before, it was deterministic equality testing of random polynomials; now it's probabilistic equality testing of random polynomials.