Minimize fuzz corpora with all fuzz engines used for fuzzing

[dergoegge]

At the moment, our script for minimizing the fuzz corpora only uses libFuzzer. Not minimizing with the same engines that we use for fuzzing will lead to coverage loss each time we minimize, because the engines differ in the way they collect coverage (e..g libFuzzer uses llvm's pc-guard but afl++ has it's own plugins).

(I need to find a source other than word of mouth on this but llvm's pc-guard apparently under reports coverage quite heavily due to missing support for instrumenting iselect expressions).

We should extend the script to also minimize with:

• afl++
• honggfuzz