Use UTC timezone for testing pre-epoch dates

The expected value differed in some newer versions of PHP (with updated timezone DBs). Sticking with UTC keeps the test more resilient to such changes, while still allowing us to deal with negative timestamps.

Author: jmikola

tests/generic/bug00667.phpt

@@ -13,7 +13,7 @@ $m = new_mongo_standalone();
 $c = $m->selectCollection(dbname(), 'bug667');
 $c->drop();
 
-$mongoDate = new MongoDate(strtotime('1900-01-01 America/New_York'));
+$mongoDate = new MongoDate(strtotime('1900-01-01 UTC'));
 $c->insert(array('date' => $mongoDate));
 
 $document = $c->findOne();
@@ -22,5 +22,5 @@ printf("%s\n", $mongoDate);
 printf("%s\n", $document['date']);
 
 --EXPECT--
-0.00000000 -2208970800
-0.00000000 -2208970800
+0.00000000 -2208988800
+0.00000000 -2208988800