Before#1869 we have not needed explicitconcurrent.NativeExecutionContext.queueInternal.helpComplete() (previouslyruntime.loop()) invocation in theNativeRPC.loop, becouse in the old testing infrastrucure we never spawned any Futures.
When porting new infra from Scala.js we've also ported some of possibly not very useful implementation details.

In Scala.js the only asynchronous RPC endpoint was used to execute the code. In Scala Nativethis endpoint was always implemented as synchronous. However, the internal implementation of the endpoint binded usingattach was delegating the execution usingattachAsync - this required us to pass the Execution context that would be used to createonComplete event.

In Scala.js infra and modified JUnitRuntime used the continuations to delay completion and notifing the JVM server until test execution has finished. It was never ported to Scala Native - the original port used a mostly synchronous variant to execute code , and last port in#2723 was rejected.

In this PR we makes synchronous RPC endpoints truly synchronous - we no longer create dummy Futures that serve little to no real purpose. It should not negatively affect the testing infrastructure - the JVM server would wait until current execution would finish before sending a next task to execute.

The benefit of this change is fact that now tests with more of the JVM semantics - if multithreading is enabled we would no longer use a ForkJoinPool/ExecutionContext to execute tests, but would execute them on main thread which is consistent with the JVM.