Tests: hop-by-hop headers in early hints responses

[SEPURI-SAI-KRISHNA]

Proposed changes

Adds proxy_early_hints_headers.t.

Companion to nginx/nginx#1604 — this test does not
pass until that change lands.

The upstream sends Connection, Content-Length, Transfer-Encoding,
Keep-Alive and Upgrade in its 103 (Early Hints) response. None of them
should reach the client: they are hop-by-hop header fields (RFC 9110, 7.6.1),
a 1xx response never carries content, and with HTTP/2 and HTTP/3 they make the
informational response malformed (RFC 9113, 8.2.2; RFC 9114, 4.2).

Covers HTTP/1.1, HTTP/2 and HTTP/3. The HTTP/1.1 check is scoped to the 103
header block only, since the final response legitimately carries
Connection and Content-Length.

Against current nginx master the test fails as follows — note that HTTP/2 and
HTTP/3 forward all five fields:

#   Failed test 'no hop-by-hop headers'
#   at proxy_early_hints_headers.t line 96.

#   Failed test 'h2 no hop-by-hop headers'
#   at proxy_early_hints_headers.t line 108.
#          got: 'connection content-length transfer-encoding keep-alive upgrade'
#     expected: ''

#   Failed test 'h3 no hop-by-hop headers'
#   at proxy_early_hints_headers.t line 123.
#          got: 'connection content-length transfer-encoding keep-alive upgrade'
#     expected: ''
# Looks like you failed 3 tests of 12.

With the fix applied:

proxy_early_hints_headers.t .. ok
All tests successful.

The test is not vacuous: if early hints stop being emitted entirely, the
early hints assertion fails first, and the HTTP/1.1 hop-by-hop check would
then be scoped to the final response headers and fail too.

Verified against a build with --with-http_v2_module --with-http_v3_module
and CryptX available, so no leg of the test is skipped.

Checklist

• I have read the contributing guidelines.
• I have signed the F5 Contributor License Agreement (CLA).
• If applicable, I have added tests that prove my fix is effective or that my feature works.
• If applicable, I have checked that any relevant tests pass after adding my changes.
• I have updated any relevant documentation (README.md and/or CHANGELOG.md).