add tests for scala sttp circe #939

Job	Run time
Build sbt/Scala client, servers (samples/client/petstore/scalaz)	48s
Build sbt/Scala client, servers (samples/client/petstore/java/okhttp-gson)	45s
Build sbt/Scala client, servers (samples/client/petstore/scala-sttp)	46s
Build sbt/Scala client, servers (samples/server/petstore/scala-finch)	23m 27s
Build sbt/Scala client, servers (samples/client/petstore/scala-sttp-circe)	40s
Build sbt/Scala client, servers (samples/server/petstore/scala-cask)	1m 28s
Build sbt/Scala client, servers (samples/server/petstore/scala-http4s-server)	52s
Build sbt/Scala client, servers (samples/client/petstore/scala-pekko)	46s
Build sbt/Scala client, servers (samples/server/petstore/scala-akka-http-server)	44s
Build sbt/Scala client, servers (samples/server/petstore/scala-pekko-http-server)	47s
Build sbt/Scala client, servers (samples/server/petstore/scala-lagom-server)	4m 4s
Build sbt/Scala client, servers (samples/server/petstore/scalatra)	3m 3s
Build sbt/Scala client, servers (samples/client/petstore/scala-http4s)	45s
Build sbt/Scala client, servers (samples/server/petstore/scala-play-server)	4m 49s
	43m 44s

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