Add shutdown acceptance test for #122

[mgrossmann]

Summary

Adds tests/shutdown-acceptance.sh, the httpd-side acceptance criterion for
the shutdown crash in #122 (epic mvslovers/mvsmf#177, Workstream D).

After a request handler faults, P HTTPD must terminate the address space
cleanly: no S33E, no repeated __CRTGET CRT for TCB(...) was not found in PPA(...), no SVC dump. The code fix lives in libc370 (worker DETACH
gating + recovery-exit hardening); this test observes the symptom in httpd's
own address space.

How it works

1. Provoke a faulting handler — ABEND_HITS requests to /abend
   (DC H'0' → S0C1 inside the worker pipeline), enough to poison the pool.
2. Issue P HTTPD (pluggable adapter — manual operator by default, cmd for CI).
3. Wait for the listener to stop.
4. Assert over the captured MVS console log.

Verdict: FAIL when the crash signatures appear (the expected result on the
current pre-fix build), PASS on clean shutdown, INCONCLUSIVE when no
handler-fault marker is present.

Correctness safeguards

• Windowed assertion. CONSOLE_LOG is typically an append-only Hercules
  hardcopy log that already contains prior S33E / __CRTGET lines. The
  script records the byte offset before provoking and asserts only over bytes
  appended during the run, so a stale crash cannot pin the result at FAIL.
• Fault marker. Proof-of-fault accepts HTTPD062E (core abend caught by
  the worker try(serve_client)) or MVSMF99E (mvsMF CGI abend caught by
  its inner ESTAE) — valid whether ABEND_PATH is a core path or a CGI endpoint.

Notes

• Expected to fail on the current build; passes only after the libc370 fix
  relinks in. A PASS on an unfixed build is flagged as suspicious in the banner.
• This PR adds the test only. Shutdown crash: S33E on worker DETACH + recovery ESTAE runs C runtime under a torn-down CRT #122 stays open as the httpd-side tracking and
  acceptance record until the libc370 side lands and this test is green.

Refs #122