Disable wxDateTime tests failing due to TZ offset changes

wxDateTime timezone-related methods always use the current timezone
offset, while other methods, using CRT, use correct value for the given
date, which may be different.

This discrepancy accounted for test failures in Europe/Minsk time zone
as Belarus has switched from UTC+2 to UTC+3 since 1999 date used in the
test.

It is impossible to really fix the problem easily, so just skip the test
in this case and also mention this bug in the documentation.

See #15370.

interface/wx/datetime.h

@@ -1248,6 +1248,10 @@ public:
         for more information about time zones. Normally, these functions should
         be rarely used.
 
+        Note that all functions in this section always use the current offset
+        for the specified time zone and don't take into account its possibly
+        different historical value at the given date.
+
         Related functions in other groups: GetBeginDST(), GetEndDST()
     */
     //@{