qa_config: Support callable test checks

Until now all tests depended on simple AND/OR conditions. With the
introduction of virtual cluster support, some tests can only run if
virtual clusters are not in use (e.g. not yet supported or not
possible). Instead of introducing more logic for enabling/disabling
tests, callable values are supported instead. “ht.WithDesc” should be
used on them to continue to provide a readable description.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>