Warn on missing SMT solvers if flag is set

[wadoon]

Currently, if SMT solvers are not available during testing, these test cases will be silently ignored---a behavior acceptable on local machines, but not on CI.

On CI, this is firstly ensured by executing z3 and cvc5 on command-line. This ensures installation, but not that KeY finds the executable.

This PR changes the behavior of SMT test cases to failing if SMT solvers are not present, and an additional flag is set.

This flag is set iff the environment variables sets failIfSmtSolverIsUnavailable or GITHUB_ACTIONS to true.

Type of pull request

• New feature (non-breaking change which adds functionality)
• There are changes to the (Java) code

Ensuring quality

• I added new test case(s) for new functionality.
• I have tested the feature as follows: CI

[wadoon]

Fails as cvc4 is not available on Github Actions. @WolframPfeifer

[wadoon]

Migration to cvc5 as far as possible.

[mattulbrich]

Agreed at meeting: Retire CVC4 support. Migrate test cases to CVC5 is possible, drop otherwise.
@WolframPfeifer can migrate test cases from other PR #3592. Then merge that onto master, that might resolve issues here.

[wadoon]

wait for #3592

[WolframPfeifer]

#3592 is merged into main, I updated this PR. Also, I migrated the CVC4 test case to cvc5, so this PR should be merge-ready now.

[WolframPfeifer]

We need an additional reviewer, since @wadoon and I both have committed to this PR ...
Can somebody else please review? It contains only small changes and should be very easy to review.