dockey_test.cc: Fix MSVC nodiscard warnings
dockey_test.cc: Fix MSVC nodiscard warnings

MSVC (with C++17) warns that the return value of make_unique<> is
ignored in this test:

    dockey_test.cc(36): warning C4834: discarding return value of function with 'nodiscard' attribute

Change-Id: I783bea2dcb712742694f8ec6264d547eba17cda1
Reviewed-on: http://review.couchbase.org/123081
Reviewed-by: Ben Huddleston <6c0d3f0e87b08a8c49786ca528f2c4b1ad3d36f3@couchbase.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine