support WEEK as a selector of EXTRACT, and refactor test cases for EXTRA...

...CT
I like this style of workaround for isHSQL better than what was there before -- testing the specific different result expected from HSQL and providing a comment explaining why it is different is an improvement, but now it is inconsistent with how other cases of HSQL differences are handled -- see the testing of "EXTRACT(WEEKDAY " just below. Please update the other tests wherever we know what to expect from HSQL (and why) to test that expectation rather than skip the test.