Really support milliseconds in wxLog

Although this was supposed to work, specifying "%l" in wxLog time stamp
format actually didn't because wxLog timestamps were stored as seconds.

Fix this by storing them as milliseconds and add a simple test (not
executed by default) showing that "%l" works correctly now.

Still keep the old wxLogRecordInfo::timestamp field for compatibility,
as it was documented.

See #13059.

samples/thread/thread.cpp

@@ -471,7 +471,7 @@ MyFrame::DoLogRecord(wxLogLevel level,
     DoLogLine
     (
         m_txtctrl,
-        wxDateTime(info.timestamp).FormatISOTime(),
+        wxDateTime(wxLongLong(info.timestampMS)).FormatISOTime(),
         info.threadId == wxThread::GetMainId()
             ? wxString("main")
             : wxString::Format("%lx", info.threadId),