accept "is null" and related syntax (Fixes #443)
- including "is not null" and "is (not) true/false"
- with a new func `IsNull`, currently just translated
(by the backend) to `= null`
- numerous tests, several pending new related tickets

Also some small improvements to the integration test driver:
- no longer loading data sets for skipped tests
- including the test file path in the name, so you can filter on it using "-ex"
reference the right data set
Merge pull request #468 from mossprescott/ready/443

accept "is null" and related syntax (Fixes #443)