Fix bug in group queries related to node/instance fields

Since we use the primitive string type for group UUIDs, the group
fields have a bug where we pass the group name as filter for node
tests, whereas the nodes themselves use the group UUID. This results
in zero node count, empty node list, and no instances being reported
as assigned to groups.

The patch fixes this and adds a test for the node count. It does some
test generation improvement, and also cleans up whitespace issues in
Test/G/Q/Query.hs (the functions case_queryNode_allfields,
prop_queryGroup_noUnknown and case_queryGroup_allfields are unchanged
but simply have indentation fixed).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>