new_category_member: Add custom test behavior

New category member tests can be flaky because they depend on recent
editing volume. This change tries to make tests more likely to pass by
looking for a special value ("test") and switching to a common
category ("All stub articles") and a 24 hour duration rather than the
normal 1 hour window.

The SYSTEM_UNDER_TEST_VALUE ("test") sentinel value introduced here
is also used retroactively in trending_topics, new_article, and
new_hashtag testing scenarios. This is hoped to make things a bit less
confusing for future maintainers than the prior overloading of the
IGNORE_VALUE ("ignore") sentinel.