Add cram tests for CFGs

[sim642]

This cherry-picks some commits from #847, but uses graph-easy to visualize the .dot output instead of testing the Graphviz code directly.

For end-to-end YAML witness location testing, #1357 is what we finally want, it's not super convenient and general to have to check for node locations through YAML witnesses. This is slightly more direct by both testing the CFG structure (which we've had problems about) and all node locations simultaneously.

TODO

• Get rid of __goblint_dummy_init, at least in these tests.
• Go back to per-function CFGs in these tests because 00-sanity/21-empty-loops output is massive only due to the big switch in main, which is orthogonal to the test.
• Fix GobView (Adapt to compute_cfg move gobview#41).