Unit tests for frontend API

[ex0dus-0x]

As we approach integrating type hints and checking with mypy (https://github.com/trailofbits/deepstate/issues/260 and https://github.com/trailofbits/deepstate/pull/268) and other fun static checking stuff into the dev cycle, it would also be nice to have a set of unit tests for front-ends as well. This way we are able to establish that basic functionality is preserved when changes are added in as we refactor the codebase towards suggestions from linters/checkers.

This might be a little tricky due to the way frontends interact greatly with the underlying environment, so using a mock testing framework may help with the efforts in doing so.

[GrosQuildu]

Implemented basic fuzzer test: simple crash and for seed synchronization. In CI, they are run inside the docker.

We may change the CI workflow to use container option instead of run: |, but that would require publishing deepstate image in public registry. Because github actions can't use local images as container argument afaik.