Add an explicit test config for use_variant.

So far the `use_variant` feature is only tested indirectly, using a
config file that still uses the old `use_base: true` mechanism (and that
is automatically migrated to `use_variant: base`). We add a test config
that directly uses `use_variant` from the start.

Author: gouttegd

tests/configs/test-use-variant.yaml

@@ -0,0 +1,20 @@
+id: tvariant
+title: "Test Variant"
+github_org: INCATools
+repo: ontology-development-kit
+import_group:
+  use_base_merging: true
+  exclude_iri_patterns:
+    - <http://purl.obolibrary.org/obo/GOCHE_*>
+    - <http://purl.obolibrary.org/obo/OBA_*>
+  slme_individuals: exclude
+  mirror_max_time_download: 400
+  products:
+    - id: ro
+      mirror_from: https://raw.githubusercontent.com/INCATools/ontology-development-kit/master/tests/ontologies/ro.owl
+    - id: pato
+      use_variant: base
+      # This test cannot be easily replaced, because use_variant and custom URL dont mix
+    - id: pr
+      make_base: true
+      mirror_from: https://raw.githubusercontent.com/INCATools/ontology-development-kit/master/tests/ontologies/pr_slim.owl