CBQE-1840: tuq - add tests for key expressions

Change-Id: If94df0be49c834e638d727fc4c6b1cd8e34a16e6
Reviewed-on: http://review.couchbase.org/31769
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>