Cleanup approval requirement testing

On further inspection, trigger requirements will still be required
for parts of the kind of pipeline construction that we are using in
OpenStack; the idea that they could be completely replaced by
pipeline requirements is incorrect.  To that end, un-deprecate
trigger requirements and cleanup the testing of both of them so that
each requirement is tested (relatively) independently.  A small
amount of requirement composition is also tested, though all possible
combinations (a lot!) are not.

Move this testing into a new file for organizational purposes.

Also, support multiple vote values (eg, "+1" or "+2") when requiring
an approval.

Change-Id: I683c9a574ced0e27ced59e62b8059fef2dfd8b20