Write an automated sanity test

[Botond Ballo]

It would be nice to be able to make changes to either the frontend or backend, and gain confidence that the changes don't break things in obvious ways without doing manual testing.

I'm thinking of writing an automated test that runs cfviz on a simple example code and checks that the expected output has been produced.

[Botond Ballo]

It makes sense to fix #4 first, since the test is both easier to write and can run on more platforms (notably, macOS) if it uses vanilla gdb rather than rr.

[Botond Ballo]

Add an automated sanity test, invoked via 'make test' (fixes #19)

→ <<cset 3192c2571d02>>