Tests: switch from DSS to ECDSA in ssl_certificates.t. All known supported platforms are shipped with OpenSSL version that supports ECDSA certificates so it's safe for a switch. Besides that, as an additional demand to switch, LibreSSL removed DSS/DSA support in 2.6.0 and nginx breaks here with such cert which is covered under try_run() which is still there. While here, now that DSS is no more, remove henceforth unneeded try_run().