fix warn_for_delay tests for v0.14

[sonots]

v0.14 TestLogger currently outputs as:

2011-01-03 13:07:36 +0000 [warn]: [] out_s3: delayed events were put

This fix is just to avoid such diff.

FYI: v0.14 TestLogger tries to add optional_header like [#{self.class.name}]

From: fluent/fluentd/lib/fluent/log.rb @ line 426 Fluent::PluginLoggerMixin#configure:

    422: def configure(conf)
    423:   super

    428:   if level = conf['@log_level']
    429:     unless @log.is_a?(PluginLogger)
    430:       @log = PluginLogger.new($log.dup)
    431:     end
    432:     @log.level = level
    433:     @log.optional_header = "[#{self.class.name}#{plugin_id_configured? ? "(" + @id + ")" : ""}] "
    434:     @log.optional_attrs = {}
    435:   end
    436: end

but, TestDriver creates plugin instance with Class.new, therefore, it does not have class name. This is reason why we get the extra [] on v0.14

[repeatedly]

Thx!