Fix test coverage related issues

[hbierlee]

On #936, a prototype test coverage tool is shown which runs in CI, and from which you can download a report which shows untested lines. So far, this uncovered the following issues worth investigating by this tracking issue. Some of these indicate missing tests, or more importantly, code which is intended to run but isn't.

• ortools no_overlap_optional decomp is never used although there seems to be a test for it: forgotten comma in supported_globals ortools #942
• MiniZinc's regular case is never used because regular is not added to its supported: Support and fix the regular constraint for MiniZinc #952
• The report seems to say the native_mus tests are never run: Test Native MUS function #950
• in mus we never seem to have an unsat case, probably just lacking the test but a bit suspicious
• couple of unused cases in GCS
• pysat checks for Comparisons twice: Remove duplicate Comparison handling #951
• our solver_vars are often checking for constants e.g. is_num(x): return x but this never seems to be the case. Maybe a bit dangerous to optimize out but could be handled last
• some cases for pumpkin also are never used, most notably mul

Additional issues can be added from time to time by updating the PR, merge in new code, and running the CI. Alternatively, you can install the pytest-cov (instruction will be in adding_solver.md / https://cpmpy.readthedocs.io/en/latest/adding_solver.html#testing-your-solver once released) to create the report locally (useful for solvers which do not run on CI e.g. gurobi)

Originally posted by @hbierlee in #936 (comment)

[hbierlee]

The original report generated in b254048 coverage-report.zip