Fix unstable Circle CI spec

[uu59]

Failures:

  1) Fluentd::Agent::LocalCommon #config_write keep files num up to max
  Failure/Error: expect(backup_files.size).to eq ::Settings.max_backup_files_count

    expected: 100
         got: 101

    (compared using ==)
  # ./spec/models/fluentd/agent/local_common_spec.rb:71:in `block (3 levels) in <top (required)>'

Sometimes our spec is failed on Circle CI as above. got: 101 is dancing about 100-110 range.