[Tests] NFC: Add a test-case for main-actor isolated struct and nonisolated protocol

Author: xedin

test/Concurrency/nonisolated_rules.swift

@@ -196,6 +196,20 @@ struct TestEnum {
   }
 }
 
+nonisolated protocol NonisolatedWithMembers {
+  func test()
+}
+
+@MainActor
+struct TestIsolated : NonisolatedWithMembers {
+  var x: NonSendable // expected-note {{property declared here}}
+
+  // requirement behaves as if it's explicitly `nonisolated` which gets inferred onto the witness
+  func test() {
+    _ = x // expected-error {{main actor-isolated property 'x' can not be referenced from a nonisolated context}}
+  }
+}
+
 // MARK: - Classes
 
 nonisolated class K: GloballyIsolated {