When using `HARNESS_JOBS` to run the tests in parallel, no matter the
level of parallelism that can be used, the monolithic `test_fuzz` takes
a long time to run, conditioning the duration of the whole build.
This commit splits the single `test_fuzz` recipe into separate recipes
for each fuzzer.
The previous mechanism to select individual fuzz tests using the
`FUZZ_TESTS` environment variable is also dropped (and documentation
updated).
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/13307)
