Correct wxDateTime::GetWeekOfMonth() for days in the beginning of January.

Correct wxDateTime::GetWeekOfMonth() for days in the beginning of January.

This function was implemented in terms of GetWeekOfYear() which made it tricky
to get it right as GetWeekOfYear() can, correctly, return week number for the
previous year for the first days of January (and also from the next one for
the last days of December).

Replace this implementation with a simple one directly counting the number of
weeks since the first of the month, this seems to be much simpler and does
pass the new unit test case which the old version failed.

Also make the unit test failures more informative by using
WX_ASSERT_EQUAL_MESSAGE() instead of CPPUNIT_ASSERT_EQUAL().

See #11561.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62916 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775