Add rotate tests in in_tail plugin
Add a test of not setting 'pos_file' paramter to in_tail plugin.
Override Engine.emit_stream for tests before input plguin start
Add read_from_head config paramter to in_tail plugin.
Add a test of setting 'read_from_head' config paramter to in_tail plugin.
Refactor CONFIG constant variable in test/plugin/in_tail.rb
Merge commit '400d95d'