Tests: RFC9218 extensible prioritization

[ac000]

These patches accompany the RFC9218 HTTP/2 extensible prioritization
implementation for nginx (posted to nginx/nginx#1520).

Patch 1 adds h2_rfc9218.t with 13 tests covering:

  - SETTINGS_NO_RFC7540_PRIORITIES (0x9) advertisement
  - Priority header parsing (u=1, u=3 i, u=5 i=?0)
  - Malformed priority header handling (defaults applied)
  - PRIORITY_UPDATE frame error cases (wrong stream, even/zero ID)
  - PRIORITY_UPDATE before HEADERS (buffered priority)
  - PRIORITY_UPDATE for existing stream
  - Upstream Priority header passthrough via proxy
  - RFC9218 Section 8 priority merge behavior

Patch 2 removes h2_priority.t since RFC7540 priority handling has been
removed from nginx in favor of RFC9218.

These patches were produced with AI assistance (Claude Opus 4.5 via
gh-copilot), as indicated by the Assisted-by tag in each commit.

 h2_priority.t | 449 ----------------------------------------------------------
 h2_rfc9218.t  | 272 +++++++++++++++++++++++++++++++++++
 2 files changed, 272 insertions(+), 449 deletions(-)
 delete mode 100644 h2_priority.t
 create mode 100644 h2_rfc9218.t

-- 

[sindhushiv]

https://github.com/nginx/nginx/issues/1423

[sindhushiv]

nginx/nginx#1420