Add check to only compile clipping tests if Z3 is installed

[AlexBork]

Add check to only compile clipping tests if Z3 is installed.

Fixes #408

[sjunges]

Thanks for the quick action.

Two questions/remarks:

• the changes seem to do more than handle the behavior when z3 is not installed. Can you make sure that the commit message or a comment here reflects that for future reference?
• wouldnt it be more adequate to let storm::test::z3AtLeastVersion(x, y, z) return false if z3 is not installed, instead of skipping them?

[AlexBork]

I reordered the tests a bit so that only one check is needed. The delta looks more complicated as there a couple of similar lines that were interpreted to not be changed, but functionally nothing besides the check changed.

Witht he solution, I followed how this is handeled for example in src/test/storm/modelchecker/multiobjective/SparseMdpPcaaMultiObjectiveModelCheckerTest.cpp. Generally, I think it's a good idea to let storm::test::z3AtLeastVersion(x, y, z) return false if z3 is not installed as that would reflect the intuitive meaning of the function. As other instances where this is used should also be guarded using the preprocessor directive, changing it should (hopefully) not create any problems. Let me know if you want me to implement that.

[sjunges]

Ok, thanks for the explanations. I see that skipping makes sense, and contrary to what I thought, the test-z3-version method is only part of the test-utilities.

LGTM :-)