Inspect roots to avoid certain resultants (Algorithm 4, lines 8,9). (#4892)

Inspect roots to avoid certain resultants (Algorithm 4, lines 8,9). (#4892)

This PR adds a small improvement to the CAD solver.
In Algorithm 4 of https://arxiv.org/pdf/2003.05633.pdf in lines 8 and 9, we only consider polynomials for resultant computations that have roots outside of the current interval.
This PR implements this check.
Fixes CVC4/cvc4-projects#210.