Reduce threshold in ext/date/tests/bug73837.phpt

Now that this code is faster, we're seeing many spurious failures.

Author: nikic

ext/date/tests/bug73837.phpt

@@ -11,8 +11,9 @@ for ( $i = 0; $i < 1000; $i++ )
     $collect[$key] = true;
 }
 
+// For low-resolution clocks, we may construct many objects in the same tick.
 var_dump($n = count( $collect ));
-echo ( $n > 700 ) ? "microseconds differ\n" : "microseconds do not differ enough ($n)\n";
+echo $n > 500 ? "microseconds differ\n" : "microseconds do not differ enough ($n)\n";
 ?>
 --EXPECTF--
 int(%d)