Add some unittests for node queries

These new tests check that:

- no known fields return unknown
- any unknown field returns unknown
- the type of the fields is consistent between the getters and the
  field definition
- the length of each result row corresponds with the number of fields
  queried, and the length of the field definitions returned
- the length of the rows corresponds to the number of nodes
- querying fields on empty fields returns all fields

Finally this patch found a bug, in that the pinst_list/sinst_list
fields were declared as QFTNumber (copy-paste error from
pinst_cnt/sinst_cnt), yay!

I also changed genEmptyCluster to ensure that it generates unique node
names, so that the number of result rows is consistent with what we
requested, and switched ResultEntry from a normal constructor to
record syntax, so that we can extract the fields without having to use
pattern matching.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>