Add new reviewers for conformance

Put into OWNERS files what has effectively been happening
for the past few months, we review and shepherd PR's
through other SIG reviewers before bringing to sig-arch
for a final /approve.

The goal is to eventually grow approvers when it's been
demonstrated that we have sufficient context.

test/conformance/testdata/OWNERS

@@ -4,6 +4,9 @@ options:
 reviewers:
   - bgrant0607
   - smarterclayton
+  - spiffxp
+  - timothysc
+  - dims
 approvers:
   - bgrant0607
   - smarterclayton