Merge pull request #491 from fluent/unit-test-unit-run

Make it possible to specify a test file to run
Conflicts:
	test/plugin/test_buf_file.rb
	test/plugin/test_buf_memory.rb
	test/plugin/test_filter_grep.rb
	test/plugin/test_in_exec.rb
	test/plugin/test_in_forward.rb
	test/plugin/test_in_gc_stat.rb
	test/plugin/test_in_http.rb
	test/plugin/test_in_object_space.rb
	test/plugin/test_in_status.rb
	test/plugin/test_in_stream.rb
	test/plugin/test_in_syslog.rb
	test/plugin/test_in_tail.rb
	test/plugin/test_in_tcp.rb
	test/plugin/test_in_udp.rb
	test/plugin/test_out_copy.rb
	test/plugin/test_out_exec.rb
	test/plugin/test_out_exec_filter.rb
	test/plugin/test_out_forward.rb
	test/plugin/test_out_roundrobin.rb
	test/plugin/test_out_stdout.rb
	test/plugin/test_out_stream.rb
	test/test_buffer.rb
	test/test_config.rb
	test/test_configdsl.rb
	test/test_match.rb