Reinstate validation proxy CI tests

Now that we have removed the OpenAPI Generator we can go ahead and
reinstate the [Prism validation proxy][1] tests, which were removed
in #1

[1]: https://meta.stoplight.io/docs/prism/docs/guides/03-validation-proxy.md

.github/workflows/tests.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
-        gauge-env: [default, mock]
+        gauge-env: [default, mock, validation-proxy]
     steps:
 
       - name: Checkout code