Add tests for scala sttp client (jsonLibrary: circe) #940

Job	Run time
Build sbt/Scala client, servers (samples/client/petstore/scala-pekko)	49s
Build sbt/Scala client, servers (samples/client/petstore/scala-sttp)	38s
Build sbt/Scala client, servers (samples/client/petstore/java/okhttp-gson)	43s
Build sbt/Scala client, servers (samples/server/petstore/scala-pekko-http-server)	46s
Build sbt/Scala client, servers (samples/client/petstore/scala-sttp-circe)	47s
Build sbt/Scala client, servers (samples/server/petstore/scala-finch)	24m 49s
Build sbt/Scala client, servers (samples/client/petstore/scala-http4s)	52s
Build sbt/Scala client, servers (samples/client/petstore/scalaz)	56s
Build sbt/Scala client, servers (samples/server/petstore/scala-http4s-server)	49s
Build sbt/Scala client, servers (samples/server/petstore/scalatra)	2m 59s
Build sbt/Scala client, servers (samples/server/petstore/scala-akka-http-server)	55s
Build sbt/Scala client, servers (samples/server/petstore/scala-cask)	1m 23s
Build sbt/Scala client, servers (samples/server/petstore/scala-play-server)	6m 1s
Build sbt/Scala client, servers (samples/server/petstore/scala-lagom-server)	3m 40s
	46m 7s

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