check if stress.c can be used for testing in python, too

[r10s]

cmdline/stress.c tests some low-level functions with some hand-written tests.

the exposes function stress_functions(dc_context_t*) is currently only called in the cmdline utility on startup, however, maybe it makes sense to call them also in the python testing context.

[ralphtheninja]

@r10s Possible to expose the test via cli, like delta --stress-test? Then we could just spawn a process from py and js.

[r10s]

closed by #568