Improve some tests using the library Truth,
which allows to write readable assertions
and provides much better error messages
than a simple call to assertTrue().
Replace usages of the similar library Hamcrest with Truth.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@14373 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c