tests: Setup defaults for subjects and artifacts

While hacking on tests, folks are going to be relying on the
default behavior of standard-test-roles which fills in these
Ansible variables from environment variables.

This is not necessary for execution in the pipeline, but just
for ease of use for folks looking at this first test in dist-git.

tests/tests.yml

@@ -1,5 +1,8 @@
 ---
 - hosts: localhost
+  vars:
+    artifacts: "{{ lookup('env', 'TEST_ARTIFACTS')|default('./artifacts', true) }}"
+    subjects: "{{ lookup('env', 'TEST_SUBJECTS')}}"
   tags:
     - atomic
   tasks: