Incremental SMT back-end occasionally fails regression tests

[tautschnig]

I have seen this more than once before, and the latest instance is https://github.com/diffblue/cbmc/actions/runs/4196248319/jobs/7276954029:

Running incremental SMT2 solving via "z3 --smt2 -in"
Invalid SMT response "cbmc"
Invalid SMT response received from solver.

I don't recall whether it is always the same test that is failing (on this occasion: array-cell-sensitivity3/test_execution.desc), but all failures I have observed were in the "codecov-coverage-report" CI job. I have not managed to reproduce this failure locally.

[tautschnig]

https://github.com/diffblue/cbmc/actions/runs/4197648661/jobs/7280244383 is another example, and it happens to be the same regression test.

[tautschnig]

Adding one more: https://github.com/diffblue/cbmc/actions/runs/4222195921/jobs/7330414616.