Test utility methods and additional test drivers

[tagomoris]

When we want to write tests of 3rd party plugins, we need some more code for:

• generating Fluent::EventTime easily
• generating Fluent::Config::Element easily (required for owned plugins - parser, formatter, storage, buffer and others)
• test drivers for owned plugins to support to
  • instantiate plugins
  • configure plugins
  • set logger, plugin id and override system configs

[tagomoris]

Code of test drivers are not proven because there are not tests for each parsers/formatters.
I'll fix these drivers when I'll add tests for these.

[cosmo0920]

It seems that last [] is needed.

-@config = Fluent::Config::Element.new(@section_name, "", Hash[conf.map{|k,v| [k.to_s, v]}])
+@config = Fluent::Config::Element.new(@section_name, "", Hash[conf.map{|k,v| [k.to_s, v]}], [])

[tagomoris]

Correct. I'll fix it.

[tagomoris]

done

[tagomoris]

Merged.