Figure out a way to test UNSAT cache

Due to improvements in #935, our previous test for UNSAT cache had to be removed, because propagating inequalities made the whole example trivial to solve.

We need a new way to test that UNSAT cache mechanism is working.

Start by reading issue #935 and inspecting the current UNSAT cache mechanism and the location where it is exercised. Identify the previous removed test or its replacement entry point, then add a test that demonstrates the cache is used without the example becoming trivial; done means the test distinguishes cached UNSAT results from ordinary propagation.

Written by the indexing model from the issue text.