test: make the example tests pass on macOS

[u5surf]

Proposed changes

Two unrelated failures in examples/t, both only visible once the functional
suite runs on a Mac, which no CI job does yet. #320 adds such a job and both
turned up there.

awssig.t writes a config using a module that is no longer built on
Darwin after #319, so nginx refuses to start. Skip the file there, matching the
configure-time condition.

shared_dict.t fails on its check() calls, subtests 3, 4 and 6. check()
records the worker pid that answered and retries back to back until a different
one answers, which is what proves the value came from shared memory rather than
worker-local state. Consecutive connections keep landing on the worker that is
already awake, so the loop exhausts without ever seeing a second pid. With
plain nginx and two workers:

worker_processes 2;
events { }
http {
    server {
        listen 127.0.0.1:18099;
        location / { return 200 "$pid\n"; }
    }
}

$ for i in $(seq 1 40); do curl -sS http://127.0.0.1:18099/; done | sort | uniq -c
  2 52118
 38 52120

Pausing between retries lets the worker that just answered go back to sleep, so
the kernel has a reason to wake the other one.

Testing

macOS 14.6, Apple Silicon, nginx 1.30.4, running shared_dict.t in a loop:

	failures
before	50 / 50
after	0 / 50

The loop returns as soon as it sees a second pid, so on a platform where the
first retry already lands on the other worker the cost is one pause per
check(). The whole of examples/t takes about 2s here either way.

accept_mutex on and listen ... reuseport were both tried first; on this
platform each made the split worse, 40 / 0.

Checklist

• I have written my commit messages in the Conventional Commits format.
• I have read the CONTRIBUTING doc
• I have added tests (when possible) that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[u5surf]

Folded into #320, which is where both failures surfaced.