Make JWATResource compliant to Resource expectations

Global Wayback now uses {{JWATFlexResourceStore}} for loading W/ARC records, but it's not fully tested until now. It turned out it did not implement special case handling, such as for {{metadata}} records and old-style revisit records. It was the root cause of WWM-126 (incorrect content-type header for metadata record replay).

This patch adds {{JWATRsourceTest}} reusing {{WarcResourceTest}}, and implements all missing special case handling to pass the test. It fixes WWM-126.  Change to {{WarcResourceTest}} is to make it reusable for {{JWATResourceTest}}. No new tests are added.