Create grouping of tests that exercise '--use-approx' (#6958)

Create grouping of tests that exercise '--use-approx' (#6958)

Currently, it seems that `cvc5` has no tests that get anywhere near close to exercising the code in `approx_simplex`, which means that the GLPK integration is effectively untested in CI:

* https://cvc4.cs.stanford.edu/downloads/builds/coverage/cvc5-2021-08-02/index.src_theory_arith_approx_simplex.cpp.html

(which is at 2.8% at the time of writing!)

Indeed, if you run the whole of `ctest`, none of the code near GLPK is exercised; and, worse, there are *no* regression tests that use `--use-approx`!

This commit adds a selection of simple tests that have been derived (using `cvc5`'s existing regression suite and ddSMT) to reach code inside of `approx_simplex`; namely:

- `solveRelaxation`, and
- `solveMIP`

This is (hopefully) an initial start that will allow us to grow more regression tests exercising GLPK (e.g., following the same process of reduction, but using the SMTLIB benchmarks vs. only `cvc5`'s regression suite).

The folder structure has been named such that I can do `ctest -R "use_approx"` to run only the tests that (should) reach GLPK.

Signed-off-by: Andrew V. Jones <andrewvaughanj@gmail.com>