fix: make the --jobs option actually work for libfuzzer

[Ekleog-NEAR]

Fixes #100.

Required for clusterfuzz support (PR incoming), as clusterfuzz actually uses forks.

[Ekleog-NEAR]

(The CI failure with AFL is I think one I already saw, and most likely unrelated to this PR)

[camshaft]

This patch seems bigger than it needs to be. Can we isolate the change to this single function?

[Ekleog-NEAR]

We could, but it would mean that literally every run would need to go through the libtest harness. I thought every runs did, but then saw in the bolero source code that there’s a BOLERO_LIBTEST_HARNESS environment variable, so I considered it’s not actually the case?

(Also, I thought maybe libfuzzer also needs to be able to run executables without going through the libtest harness, eg. in the dataflow things, but I have literally no idea how those works so I wrote the patch only so it doesn’t newly break stuff, at the expense of maybe not fixing them if they were already broken)

[Ekleog-NEAR]

That said if bolero fuzzers always run through the libtest harness I could at least slightly simplify the patch :)

[camshaft]

I've opened #176 as a more minimal change. The BOLERO_LIBTEST_HARNESS environment variable is just to detect if we're using libtest or not. If we're not, it would look something like:

fn main() {
    bolero::check!().for_each(|_| { });
}

Which means we still go through bolero's harness. But it'll only execute a single harness in this mode.

[Ekleog-NEAR]

Superseded by #176