Always use fixed TIME as session base in TestDateTimeFunctions

Using this will allow to write simpler tests and avoid intermittent
error caused by DST or changes in time zone policies.

Without this change some test would require to rewrite to much logic
from implementation making them less usefull.

presto-main/src/test/java/com/facebook/presto/operator/scalar/TestDateTimeFunctionsBase.java

@@ -114,6 +114,7 @@ protected TestDateTimeFunctionsBase(boolean legacyTimestamp)
         super(testSessionBuilder()
                 .setSystemProperty("legacy_timestamp", String.valueOf(legacyTimestamp))
                 .setTimeZoneKey(TIME_ZONE_KEY)
+                .setStartTime(new DateTime(2017, 04, 01, 12, 34, 56, 789, UTC_TIME_ZONE).getMillis())
                 .build());
         TIMESTAMP = legacyTimestamp ? LEGACY_TIMESTAMP : NEW_TIMESTAMP;
     }