test_out_buffer: fix flaky BufferOutputTest#write

[Watson1978]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:
out_buffer flushes its buffer immediately and re-emits events asynchronously via the flush thread.
The test driver's wait_flush_completion only waits until the chunk leaves the buffer queue , not until write() actually emits.
When the flush thread is still between dequeue_chunk and write, run() proceeds to shutdown, where EventEmitter#after_shutdown nil-s out @router before Output#after_shutdown joins the flush thread.
The in-flight write then emits to a nil router and the event is dropped, making d.events empty. This surfaces as a flaky failure under CI load.

Pass expect_emits: 1 so run() waits until the event is actually emitted (while the router is still valid) before shutting the plugin down.

This PR should fix following error:

1) Failure: test: #write(BufferOutputTest)
/Users/runner/hostedtoolcache/Ruby/3.2.11/arm64/bin/bundle:25:in `load'
/Users/runner/work/fluentd/fluentd/test/plugin/test_out_buffer.rb:52:in `block in <class:BufferOutputTest>'
/Users/runner/hostedtoolcache/Ruby/3.2.11/arm64/bin/bundle:25:in `<main>'
     49:       d.feed(time, record)
Tasks: TOP => test => base_test
     50:     end
(See full trace by running task with --trace)
     51: 
  => 52:     assert_equal [["test", time, record]], d.events
     53:   end
     54: end
<[["test", 2026-07-16 07:03:01.673742000 +0000, {"message"=>"test"}]]> expected but was
<[]>
diff:
? [["test", 2026-07-16 07:03:01.673742000 +0000, {"message"=>"test"}]]
Error: <[["test", 2026-07-16 07:03:01.673742000 +0000, {"message"=>"test"}]]> expected but was
<[]>.

Ref. https://github.com/fluent/fluentd/actions/runs/29478564922/job/87556893864#step:6:5358

Docs Changes:
N/A

Release Note:
N/A