Hypofuzz lacks setup hooks of _any_ sort

[tarunik]

Right now, Hypofuzz provides no hooks for when it starts running a test target. This is a problem because the only time prior to an individual fuzz run one can do setup work is at import time, which makes it nearly impossible to not do that work when running the testsuite conventionally (i.e. with pytest and Hypothesis, not Hypofuzz).

At the very least, a hook called from FuzzProcess.startup would let me do Hypofuzz-only setup steps instead of having to try to detect whether we're running under pytest or not during import, which may not work anyway since Hypofuzz delegates test collection (and importing) to pytest.

[tarunik]

Update: I was able to PoC this by manually monkeypatching FuzzProcess.startup, which'll get me going for now, but exposing a hook API of some sort would be appreciated.