Improve compute_pvals_perm function

[qbarthelemy]

This PR replaces

        perm = rng.randint(2, size=(data.shape[0],))
        perm[perm == 0] = -1

by

        perm = rng.choice([1, -1], size=(data.shape[0],), replace=True)

and renames "exhaustive test" by "exact test".