Get some CI going

[domenukk]

Right now, we don't have any CI for LibAFL.
However, we do have unittests and examples that can pose as test cases. These should be added to Travis, similar to AFL++.

[andreafioraldi]

libfuzzer harnesses are also goodies that I want

[vanhauser-thc]

libfuzzer harnesses are also goodies that I want

why can I only give one thumb up? bug :)

desperately needed (both, more thumbs up and libfuzzer harness)

on the libfuzzer harness: llvm 12 has new features that will be an issue to implement on our own, maybe we can leverage the functionality from llvm itself (fuzzeddataprovider)