test-qmp-client: run mock QMP servers as fibers on the shared event loop

test-qmp-client: run mock QMP servers as fibers on the shared event loop

The mock servers used to be driven out-of-band: each test created a
socketpair, forked a child, ran a hand-coded request/response script
against the raw fd, and sent SIGTERM to tear it down. That worked but
required pidref/process-util/signal plumbing in every test, two
distinct execution contexts that couldn't share state, and a JsonStream
attached to the mock side that pretended to be event-loop-driven while
actually being driven manually via blocking reads.

Now that JsonStream suspends when on a fiber, the mocks can live
inside the same process and event loop as the client. Each mock is
rewritten as an sd-fiber that runs alongside the client fiber: so the
mock fiber yields on I/O and the event loop schedules the client in the
meantime. Both sides progress cooperatively, no fork/SIGTERM/PID tracking,
no manual phase tracking.

Two cleanups fall out of the rewrite:

- A QMP_TEST(name, mock_fn) { ... } macro encapsulates the per-test
  scaffolding (event loop, socketpair, mock fiber spawn, exit-on-idle
  shim) and injects an already-connected QmpClient *client into the
  test body. Each test now reads as a flat sequence of
  qmp_client_call() invocations against that client.

- Repeated mock command/reply scripting is factored into
  mock_qmp_expect(), mock_qmp_reply(), mock_qmp_expect_and_reply(),
  mock_qmp_handshake(), and mock_qmp_query_status_running(). The
  greeting JSON is built with sd_json_buildo() instead of being parsed
  from a literal.

The file shrinks from 756 to 494 lines, mostly through deletions.