[fvectortest] Don't use assert() in tests.
Here I'm replacing assertion failure by throwing an exception.
Found by the noassert build, see https://gitlab.dune-project.org/core/dune-common/-/jobs/85368
Here I'm replacing assertion failure by throwing an exception.
Found by the noassert build, see https://gitlab.dune-project.org/core/dune-common/-/jobs/85368