fuzz: unify the "fuzz-regressions" directory with the main corpus

fuzz: unify the "fuzz-regressions" directory with the main corpus

There isn't really much need to keep them separate. Anything which is a good
corpus entry can be used as a smoke test, and anything which which is a
regression test can just as well be inserted into the corpus.

The only functional difference from this patch (apart from different paths in
output) is that the regression tests are now zipped together with the rest of
the corpus.

$ meson configure build -Dslow-tests=true && ninja -C build test
...
307/325 fuzz-dns-packet:issue-7888:address      OK       0.06 s
308/325 fuzz-dns-packet:oss-fuzz-5465:address   OK       0.04 s
309/325 fuzz-journal-remote:crash-5a8f03d4c3a46fcded39527084f437e8e4b54b76:address  OK       0.07 s
310/325 fuzz-journal-remote:crash-96dee870ea66d03e89ac321eee28ea63a9b9aa45:address  OK       0.05 s
311/325 fuzz-journal-remote:oss-fuzz-8659:address  OK       0.05 s
312/325 fuzz-journal-remote:oss-fuzz-8686:address  OK       0.07 s
313/325 fuzz-unit-file:oss-fuzz-6884:address    OK       0.06 s
314/325 fuzz-unit-file:oss-fuzz-6885:address    OK       0.05 s
315/325 fuzz-unit-file:oss-fuzz-6886:address    OK       0.05 s
316/325 fuzz-unit-file:oss-fuzz-6892:address    OK       0.05 s
317/325 fuzz-unit-file:oss-fuzz-6897:address    OK       0.05 s
318/325 fuzz-unit-file:oss-fuzz-6897-evverx:address  OK       0.06 s
319/325 fuzz-unit-file:oss-fuzz-6908:address    OK       0.07 s
320/325 fuzz-unit-file:oss-fuzz-6917:address    OK       0.07 s
321/325 fuzz-unit-file:oss-fuzz-6977:address    OK       0.13 s
322/325 fuzz-unit-file:oss-fuzz-6977-unminimized:address  OK       0.12 s
323/325 fuzz-unit-file:oss-fuzz-7004:address    OK       0.05 s
324/325 fuzz-unit-file:oss-fuzz-8064:address    OK       0.05 s
325/325 fuzz-unit-file:oss-fuzz-8827:address    OK       0.52 s