Add tests for anyof with discriminator #778

Job	Run time
Build sbt/Scala client, servers (samples/client/petstore/java/okhttp-gson)	35s
Build sbt/Scala client, servers (samples/client/petstore/scalaz)	5m 25s
Build sbt/Scala client, servers (samples/client/petstore/scala-pekko)	40s
Build sbt/Scala client, servers (samples/server/petstore/scala-lagom-server)	22m 3s
Build sbt/Scala client, servers (samples/server/petstore/scala-play-server)	6m 0s
Build sbt/Scala client, servers (samples/server/petstore/scala-akka-http-server)	2m 3s
Build sbt/Scala client, servers (samples/server/petstore/scala-pekko-http-server)	44s
Build sbt/Scala client, servers (samples/server/petstore/scalatra)	3m 44s
Build sbt/Scala client, servers (samples/server/petstore/scala-finch)	11m 36s
Build sbt/Scala client, servers (samples/server/petstore/scala-http4s-server)	48s
Build sbt/Scala client, servers (samples/server/petstore/scala-cask)	1m 1s
	54m 39s

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