Add OCSP API test executable

Some of the OCSP APIs (such as the recently added OCSP_resp_get0_signer)
do not really merit inclusion in the ocsp(1) utility, but we should still
have unit tests for them.

For now, only test OCSP_resp_get0_signer(), but it should be easy to
add more tests in the future.

Provide an X509 cert and private key in the test's data directory
to use for signing responses, since constructing those on the fly
is more effort than is needed.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4573)