Fixes for conformance generator tags and test selection

Incorrectly used "tag" in test.desc instead of "tags"
Incorrectly selected all tests rather than just all positive
tests depending on operator selection option.

Signed-off-by: Jeremy Johnson <jeremy.johnson@arm.com>
Change-Id: Ib257bc57accf745beedfa4a3b2ece9539c0972c2