test_rehash does nothing, have it do something

test/recipes/40-test_rehash.t uses test files from certs/demo, which
doesn't exist any longer.  Have it use PEM files from test/ instead.

Because rehash wants only one certificate or CRL per file, we must
also filter those PEM files to produce test files with a single object
each.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2594)