[Distributed][CustomRuntimeAttrs] Add distributed actor test cases

Making sure we can use custom runtime attributes on a `distributed actor` even if only on distributed methods.

It seems we can't do this on non distributed methods, as the call is made cross actor so the distributed isolation prevents it. Not sure if that should be possible anyway -- as long as distributed funcs can be annotated the most important part works <3

Author: ktoso

test/type/runtime_discoverable_attrs.swift

@@ -5,6 +5,7 @@
 // REQUIRES: asserts
 
 import RuntimeAttrs
+import Distributed
 
 @runtimeMetadata
 struct Flag<T> {
@@ -305,6 +306,35 @@ extension InvalidConformanceTest2 : Flagged & EnumFlagged {}
 // expected-note@-3 {{protocol 'Flagged' requires reflection metadata attribute @Flag}}
 // expected-note@-4 {{protocol 'EnumFlagged' requires reflection metadata attribute @EnumFlag}}
 
+typealias DefaultDistributedActorSystem = LocalTestingDistributedActorSystem
+
+distributed actor TestDistributedActor {
+  @FlagForAsyncFuncs distributed func asyncExternallyDist() throws {}
+
+  @FlagForAsyncFuncs distributed func doSomethingDist() async throws {}
+
+  @FlagForAsyncFuncs nonisolated func doSomethingNonisolated(_: Int) async {}
+
+  @FlagForAsyncFuncs distributed func doSomethingDist(_: Int, x: [Int]) async {}
+}
+
+@ActorFlag
+distributed actor TestDistributedActor2 {}
+
+@runtimeMetadata
+enum ActorFlag<A> {
+  case actor(A.Type)
+  case distributedActor(A.Type)
+}
+
+extension ActorFlag where A: Actor {
+  init(attachedTo: A.Type) { self = .actor(attachedTo) }
+}
+extension ActorFlag where A: DistributedActor {
+  init(attachedTo: A.Type) { self = .distributedActor(attachedTo) }
+}
+
+
 @Flag
 struct ValidConformance1 {}
 extension ValidConformance1 : Flagged {} // Ok