Fix an unstable test of in_tail (especially on macOS)

[ashie]

Which issue(s) this PR fixes:
none

What this PR does / why we need it:
This commit try to fix the following failure:

  2021-04-02T00:47:45.1133120Z Failure: test_truncate_file(TailInputTest)
  2021-04-02T00:47:45.1140150Z /Users/runner/work/fluentd/fluentd/test/plugin/test_in_tail.rb:606:in `test_truncate_file'
  2021-04-02T00:47:45.1140850Z      603:     end
  2021-04-02T00:47:45.1141130Z      604:
  2021-04-02T00:47:45.1141450Z      605:     events = d.events
  2021-04-02T00:47:45.1141920Z   => 606:     assert_equal(3, events.length)
  2021-04-02T00:47:45.1142440Z      607:     assert_equal({"message" => "test3"}, events[0][2])
  2021-04-02T00:47:45.1142960Z      608:     assert_equal({"message" => "test4"}, events[1][2])
  2021-04-02T00:47:45.1143490Z      609:     assert_equal({"message" => "test1"}, events[2][2])
  2021-04-02T00:47:45.1143940Z <3> expected but was
  2021-04-02T00:47:45.1144370Z <1>

sleep time 1sec may too short when inotify isn't available or high load situation.

Docs Changes:
none

Release Note:
none

[ashie]

It seems become stable.