Tests: removed multiple server certificates from ssl_ocsp.t. Multiple server certificates are not needed to test OCSP verification of client certificates (in contrast to OCSP stapling, where server certificates are verified, and different staples should be correctly returned with different server certificates). And using multiple server certificates causes issues when testing with LibreSSL due to broken sigalgs-based server certificate selection in LibreSSL with TLSv1.3. Accordingly, the test is simplified to do not use multiple server certificates.