add yaml test

tests/yaml/conflict_2.yml

@@ -0,0 +1,28 @@
+# Tzu-ping mentioned this example
+base:
+  available:
+    - name: virtualenv
+      version: 20.0.2
+      depends: ['six>=1.12.0,<2']
+    - six 1.11
+    - six 1.12
+    - six 1.13
+
+cases:
+-
+  request:
+    - install: virtualenv
+  response:
+    - state:
+      - six 1.13
+      - virtualenv 20.0.2
+-
+  request:
+    - install: ['six<1.12', 'virtualenv==20.0.2']
+  response:
+    - state: null
+  skip: true
+  # -- currently the error message is:
+  # Could not find a version that satisfies the requirement six<1.12
+  # Could not find a version that satisfies the requirement six<2,>=1.12.0 (from virtualenv)
+  # No matching distribution found for six, six