restrict runtime QoS introspection tests to Darwin

Tests/ReactiveSwiftTests/SchedulerSpec.swift

@@ -6,6 +6,9 @@
 //  Copyright (c) 2014 GitHub. All rights reserved.
 //
 
+#if canImport(Darwin)
+import Darwin.sys.qos
+#endif
 import Dispatch
 import Foundation
 
@@ -265,6 +268,11 @@ class SchedulerSpec: QuickSpec {
                 it("should propagate QoS values by default") {
                     expect(scheduler.queue.qos).to(equal(.unspecified))
 
+                    // qos_class_self() may not be available on non-Darwin
+                    // platforms, and it's unclear if QoS propagation is
+                    // implemented in an equivalent manner in such contexts,
+                    // so we restrict runtime validation tests to Darwin.
+                    #if canImport(Darwin)
                     let userInitiatedQueue = DispatchQueue(
                         label: "reactiveswift.tests.user-initiated",
                         qos: .userInitiated
@@ -293,6 +301,7 @@ class SchedulerSpec: QuickSpec {
                     expect(endQoS?.rawValue).to(beGreaterThanOrEqualTo(
                         initialQoS?.rawValue
                     ))
+                    #endif  // canImport(Darwin)
                 }
 			}
 		}