Use "datetime" tag for all wxDateTime unit tests

Make it possible to run all these tests at once (and no other ones)
easily.

tests/datetime/datetimetest.cpp

@@ -262,11 +262,7 @@ private:
     wxDECLARE_NO_COPY_CLASS(DateTimeTestCase);
 };
 
-// register in the unnamed registry so that these tests are run by default
-CPPUNIT_TEST_SUITE_REGISTRATION( DateTimeTestCase );
-
-// also include in its own registry so that these tests can be run alone
-CPPUNIT_TEST_SUITE_NAMED_REGISTRATION( DateTimeTestCase, "DateTimeTestCase" );
+wxREGISTER_UNIT_TEST_WITH_TAGS(DateTimeTestCase, "[datetime]");
 
 // ============================================================================
 // implementation