Use wxDateTime::TimeZone::IsLocal() in the unit test

No real changes, as this doesn't affect this test, but use the new
IsLocal() method instead of comparing the time zone offset with the time
zone, which doesn't work correctly for BST.

tests/datetime/datetimetest.cpp

@@ -722,7 +722,7 @@ void DateTimeTestCase::TestTimeFormat()
     for ( unsigned idxtz = 0; idxtz < WXSIZEOF(timeZonesOffsets); ++idxtz )
     {
         wxDateTime::TimeZone tz(timeZonesOffsets[idxtz]);
-        const bool isLocalTz = tz.GetOffset() == -wxGetTimeZone();
+        const bool isLocalTz = tz.IsLocal();
 
         for ( size_t d = 0; d < WXSIZEOF(formatTestDates); d++ )
         {