tests: add libFuzzer based fuzzing

tests: add libFuzzer based fuzzing

LibFuzzer is in-process, coverage-guided, evolutionary fuzzing engine.

LibFuzzer is linked with the library under test, and feeds fuzzed inputs
to the library via a specific fuzzing entrypoint (aka "target
function"); the fuzzer then tracks which areas of the code are reached,
and generates mutations on the corpus of input data in order to maximize
the code coverage.

So lets use libFuzzer to fuzz dns_handle_packet for the start.

Ref: https://llvm.org/docs/LibFuzzer.html
Signed-off-by: Petr Štetiar <ynezz@true.cz>