Restructure SolverTest:
- extract utility class for solver-based tests
- split SolverTest in several files
- move general tests from Z3Test into it
- use Truth to write asserts with helpful messages
  (e.g., showing a counterexample when a formula is unexpectedly
  satisfiable).
Also change PathFormulaManagerTest
to use the SolverBasedTest0 utility class.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@14603 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c