test: Fix random test failures at test/plugin/test_output.rb#L926

[fujimotos]

This patch fixes the test failures around invoke_slow_flush_log_thread_test() which
have been occurring at Travis CI.

For actual failing test runs, see job#4160.1, job#4169.4 and job#4154.5

Problem

I can reproduce this failure reliably by inserting sleep 3 into test_output.rb:

def invoke_slow_flush_log_threshold_test(i)
  ...
  i.start
  i.after_start

  sleep 3

  t = event_time()
  i.emit_events('tag', Fluent::ArrayEventStream.new([ [t, {"key" => "value1"}],
                                              [t, {"key" => "value2"}] ]))
  i.force_flush

  waiting(4) { Thread.pass until i.test_finished? }

So it is a multi-threading bug. In particular, if the buffer-flushing thread interrupts the
test execution at the code point, it will accidentally turn on the @test_finished flag
and breaks the precondition for the test case. This is the exact cause of the test failure.

Solution

This patch resolves this issue by setting a sufficiently long value to flush_interval
(30 sec) so that it won't mess with the test execution.

[repeatedly]

Thanks!