Tests: added add_header/add_trailer field sanitization test.

[louzt]

Summary

This updates the standalone nginx-tests regression for add_header and add_trailer sanitization.

Why

The companion test now covers the maintainer concern raised in review: invalid static field names in add_header/add_trailer should not be serialized either.

That keeps the test aligned with the direct upstream C draft instead of only checking CRLF in values.

What changed

• keeps the existing add_header/add_trailer CRLF value cases;
• adds bad-name cases for add_header and add_trailer using a valid sibling field plus an invalid X:Bad field name;
• verifies that the valid field still appears while the invalid one is skipped;
• keeps the usual no alerts and no sanitizer errors checks from Test::Nginx.

Validation

• current nginx/nginx master: header_filter_sanitize.t fails 4/14;
• patched branch for the companion C change: header_filter_sanitize.t passes 14/14;
• ASAN/UBSAN build of the patched branch: header_filter_sanitize.t passes 14/14.

Risks / Follow-ups

This test intentionally follows the current draft policy: sanitize bad values in place via a copy and skip bad names entirely. If upstream chooses a different late response-filter policy, the assertions should be updated to match that final design.

[louzt]

Companion nginx/nginx draft: nginx/nginx#1414