Faster fuzz test: teach the fuzz test programs to handle directories

Instead of invoking the fuzz test programs once for every corpora
file, we invoke them once for each directory of corpora files.  This
dramatically reduces the number of program invikations, as well as the
time 99-test_fuzz.t takes to complete.

fuzz/test-corpus.c was enhanced to handle directories as well as
regular files.

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5776)