Improve test stability

[tagomoris]

To make tests stable on CI services.

Based on #1425, and fixes #1404.

[tagomoris]

Currently I'm willing not to fix these 2 failures sometimes occurs.

• test_child_process
  • test to kill child process by SIGQUIT
  • child process sometimes exits with SIGSEGV
  • child process sometimes exits with return code 1, without signal info
  • https://travis-ci.org/fluent/fluentd/jobs/195464389
• test_in_tail
  • test about file rotation
  • recorded here: in_tail: tests about file rotation and deletion sometimes fail #1434

[tagomoris]

I'm expecting that this work is almost done.
I'll merge this change after 10x continuous CI Green (or Red only with known patterns).

[tagomoris]

3 (in 10) failures on CI (all on Travis CI), with 5 job. Known 2 (test_child_process about kill), unknown 3.
It can be tested by command line: bundle exec rake test TESTOPTS=-v TEST=test/{plugin/test_in_dummy.rb,plugin/test_output_as_buffered_retries.rb,plugin/test_output_as_buffered.rb,plugin_helper/test_child_process.rb}

DummyTest: 
test: value of auto increment key is not suspended after stop-and-start:F
Failure: test: value of auto increment key is not suspended after stop-and-start(DummyTest::doesn't suspend internal counters in default)
/home/travis/build/fluent/fluentd/test/plugin/test_in_dummy.rb:129:in `block (2 levels) in <class:DummyTest>'
     126:       end
     127: 
     128:       first_id2 = d2.events.first[2]['id']
  => 129:       assert_equal 0, first_id2
     130: 
     131:       assert !File.exist?(File.join(TEST_PLUGIN_STORAGE_PATH, 'json', 'test-01.json'))
     132:     end
<0> expected but was
<1>

BufferedOutputRetryTest: 
test: retry_timeout and retry_max_times will be ignored if retry_forever is true for exponential backoff:
No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.

BufferedOutputTest: 
 test: #write is called every time for each emits, and buffer chunk is purged:F
Failure: test: #write is called every time for each emits, and buffer chunk is purged(BufferedOutputTest::buffered output feature without any buffer key, flush_mode: immediate)
/Users/travis/build/fluent/fluentd/test/plugin/test_output_as_buffered.rb:756:in `block (3 levels) in <class:BufferedOutputTest>'
     753:           sleep 0.1 until @i.buffer.queue.size == 0 && @i.buffer.dequeued.size == 0
     754:         end
     755: 
  => 756:         assert_equal rand_records, ary.size
     757:         ary.reject!{|e| true }
     758:       end
     759:     end
/Users/travis/build/fluent/fluentd/test/plugin/test_output_as_buffered.rb:742:in `times'
/Users/travis/build/fluent/fluentd/test/plugin/test_output_as_buffered.rb:742:in `block (2 levels) in <class:BufferedOutputTest>'
<5> expected but was
<0>

ChildProcessTest: 
 test: can return exit status with signal code for child process killed by signal using on_exit_callback:F
Failure: test: can return exit status with signal code for child process killed by signal using on_exit_callback(ChildProcessTest::on_exit_callback is specified)
/home/travis/build/fluent/fluentd/test/plugin_helper/test_child_process.rb:683:in `block (2 levels) in <class:ChildProcessTest>'
     680:       # This test sometimes fails on TravisCI
     681:       #    with [nil, 11] # SIGSEGV
     682:       # or with [1, nil]  # ???
  => 683:       assert_equal [nil, 3, true, ""], [exit_status.exitstatus, exit_status.termsig, File.exist?(@temp_path), str] # SIGQUIT
     684:       # SIGSEGV looks a kind of BUG of ruby...
     685:     end
     686: 
<[nil, 3, true, ""]> expected but was
<[1, nil, true, ""]>
diff:
? [1, nil, 3, true, ""]

Failure: test: can return exit status with signal code for child process killed by signal using on_exit_callback(ChildProcessTest::on_exit_callback is specified)
/home/travis/build/fluent/fluentd/test/plugin_helper/test_child_process.rb:683:in `block (2 levels) in <class:ChildProcessTest>'
     680:       # This test sometimes fails on TravisCI
     681:       #    with [nil, 11] # SIGSEGV
     682:       # or with [1, nil]  # ???
  => 683:       assert_equal [nil, 3, true, ""], [exit_status.exitstatus, exit_status.termsig, File.exist?(@temp_path), str] # SIGQUIT
     684:       # SIGSEGV looks a kind of BUG of ruby...
     685:     end
     686: 
<[nil, 3, true, ""]> expected but was
<[nil, 11, true, ""]>
diff:
? [nil, 3 , true, ""]
?       11           

[tagomoris]

There are no failures on TravisCI in 20 times CI (AppVeyor is still running).
I'll re-edit commits and merge into master.

[tagomoris]

Greeeeeeeen!