Fix broken test for utils.solver with symengine

[cdiener]

• [X] fix failing test on newer sympy versions
• [X] description of feature/fix
• [X] tests added/passed
• [X] add an entry to the next release

Minor change that repairs a broken test. Also, the logic of the test was a bit off and has now been corrected.

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: +0.09% :tada:

Comparison is base (e48bc84) 83.85% compared to head (19ba0fc) 83.94%.

:exclamation: Current head 19ba0fc differs from pull request most recent head f5b8059. Consider uploading reports for the commit f5b8059 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## devel #1340 +/- ## ========================================== + Coverage 83.85% 83.94% +0.09% ========================================== Files 66 66 Lines 5499 5499 Branches 1257 1257 ========================================== + Hits 4611 4616 +5 + Misses 572 567 -5 Partials 316 316 ``` [see 1 file with indirect coverage changes](https://app.codecov.io/gh/opencobra/cobrapy/pull/1340/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opencobra)

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[cdiener]

Only affects symengine, apparently.