dbms: added test [#METR-16722].

dbms/tests/queries/0_stateless/00168_buffer_defaults.reference

+EventDate	Date		
+UTCEventTime	DateTime		
+MoscowEventDate	Date	DEFAULT	toDate(UTCEventTime)
+EventDate	Date		
+UTCEventTime	DateTime		
+MoscowEventDate	Date	DEFAULT	toDate(UTCEventTime)
+2015-06-09	2015-06-09 01:02:03	2015-06-09
+2015-06-09	2015-06-09 01:02:03	2015-06-09
+2015-06-09	2015-06-09 01:02:03	2015-06-09

dbms/tests/queries/0_stateless/00168_buffer_defaults.sql

+DROP TABLE IF EXISTS test.mt;
+DROP TABLE IF EXISTS test.mt_buffer;
+CREATE TABLE test.mt (EventDate Date, UTCEventTime DateTime, MoscowEventDate Date DEFAULT toDate(UTCEventTime)) ENGINE = MergeTree(EventDate, UTCEventTime, 8192);
+CREATE TABLE test.mt_buffer AS test.mt ENGINE = Buffer(test, mt, 16, 10, 100, 10000, 1000000, 10000000, 100000000);
+DESC TABLE test.mt;
+DESC TABLE test.mt_buffer;
+INSERT INTO test.mt (EventDate, UTCEventTime) VALUES ('2015-06-09', '2015-06-09 01:02:03');
+SELECT * FROM test.mt_buffer;
+INSERT INTO test.mt_buffer (EventDate, UTCEventTime) VALUES ('2015-06-09', '2015-06-09 01:02:03');
+SELECT * FROM test.mt_buffer;
+DROP TABLE test.mt_buffer;
+DROP TABLE test.mt;