tests: finally fix up test-fuzz-targets.t It's been failing on my workstation for a while, since I have a new enough LLVM that I had the fuzzer goo, but not so new that I actually had FuzzedDataProvider. This is a better solution all around in my opinion. I _believe_ this should let us run these tests on most systems, even those using GCC instead of clang. That said, my one attempt to test this on my macOS laptop failed miserably, and I don't feel like doing more work on this right now. Differential Revision: https://phab.mercurial-scm.org/D7566