tests: Add runner for AFL corpus.

Contributed by Jeff Epler (@jepler):

run-afl-corpus: Avoid locale-dependent filename sorting

.. by ensuring that the "C" locale is used while running this script.
This requires sorting the "ref" file which was generated using the
collation order of some other locale such as en_US.UTF-8.

Apparently, POSIX promises that shell globs use the locale's collation
ordering, but bash is alone (among dash, mksh and zsh) at actually doing
this.  For discussion, see
https://serverfault.com/questions/122737/in-bash-are-wildcard-expansions-guaranteed-to-be-in-order

Without this change, I get a failure when I "sh run-afl-test-corpus.sh"
(sh is invoking /bin/dash).  With this change, I get success with all
4 sh implementations I tested and regardless of my LANG setting
such as LANG=en_US.UTF-8 LANG=zh_CN.UTF-8 LANG=C.UTF-8 LANG=C