Start Akka Http server in one terminal tab:
sbt
rpcJVM/run
Test failing MacrotaskExecutor implicit in another tab:
sbt -client
project rpcJS
test
Switch execution context by uncommenting/commenting imports in PostRequest to:
import scala.concurrent.ExecutionContext.Implicits.global
//import org.scalajs.macrotaskexecutor.MacrotaskExecutor.Implicits._