Test Updates to Align with Header Removal from APIcast Midstream

[cathal-bailey]

• APIcast midstream update removes the Server header from HTTP responses. This breaks tests that rely on checking headers to confirm APIcast is responding.
• Update to the backoff mechanism to look for valid response code rather than server header. Ensures backoff logic remains in place and working, in case it is needed.
• test_do_not_send_openresty_version.py‎ also affected by midstream change.
• Test now verifies that no server header or version information is disclosed anywhere.