test_out_exec: Try to fix a randomly failed test

[ashie]

Which issue(s) this PR fixes:
None

What this PR does / why we need it:
I found that a test for out_exec plugin randomly fails on CI.
It often occurrs on Dron CI but sometimes on other environments too.

e.g.) https://cloud.drone.io/fluent/fluentd/1934/1/2

Failure: test: flushed chunk will be committed after child process successfully exits(ExecOutputTest::when executed process dies unexpectedly)
/drone/src/test/plugin/test_out_exec.rb:279:in `block (2 levels) in <class:ExecOutputTest>'
     276:       expect_data =
     277:         %[2011-01-02 13:14:15\ttest\tv1\n] +
     278:         %[2011-01-02 13:14:15\ttest\tv2\n]
  => 279:       assert_equal expect_data, data
     280:
     281:       assert{ d.instance.buffer.queue.empty? }
     282:       assert{ d.instance.dequeued_chunks.empty? }
<"2011-01-02 13:14:15\ttest\tv1\n" + "2011-01-02 13:14:15\ttest\tv2\n">(UTF-8) expected but was
<"">(ASCII-8BIT)

diff:
- 2011-01-02 13:14:15	test	v1
- 2011-01-02 13:14:15	test	v2

? Encoding: UTF  -8
?           ASCII  BIT
?           ???  +++

Although wait_flush_completion: false was specified, it seems that the
test expects flushing the buffer.

Docs Changes:
None

Release Note:
Same with the title

[ashie]

I tried it on CI about 10 times and the test doesn't fail anymore.

[cosmo0920]

Nice work!