test(cli): make dispatch cancellation handshake deterministic

[wgqqqqq]

Summary

• keep the test leader blocked in the shell builtin wait
• let SIGTERM run the exit trap immediately instead of waiting on an external sleep
• preserve the TERM-resistant child that verifies SIGKILL is not sent after leader exit

Why

The macOS CLI job in #2495 intermittently failed cancellation_does_not_escalate_after_term_exits_the_verified_leader. Under load, /bin/sh can defer a trapped signal while waiting for an external foreground command, so the test leader sometimes survived the one-second TERM grace window and was legitimately escalated.

Verification

• target test: 50/50 repeated passes on macOS
• all 6 dispatch::runner tests pass
• cargo check -p bitfun-cli
• cargo test -p bitfun-cli (685 unit tests plus CLI integration suites)

Only the test fixture changes; production cancellation behavior is unchanged.

[wgqqqqq]

Superseded by the root-cause production fix merged in #2497. That change rejects macOS process states carrying the exiting E modifier before SIGKILL escalation; this test-fixture-only PR is no longer the right fix.