[Concurrency] Update tests for Linux

Author: DougGregor

test/Concurrency/Runtime/async_task_priority_basic.swift

@@ -6,6 +6,12 @@
 
 import Dispatch
 
+#if canImport(Darwin)
+import Darwin
+#elseif canImport(Glibc)
+import Glibc
+#endif
+
 // ==== ------------------------------------------------------------------------
 // MARK: "Infrastructure" for the tests
 

test/Concurrency/Runtime/basic_future.swift

@@ -7,6 +7,12 @@
 
 import Dispatch
 
+#if canImport(Darwin)
+import Darwin
+#elseif canImport(Glibc)
+import Glibc
+#endif
+
 extension DispatchQueue {
   func async<R>(execute: @escaping () async throws -> R) -> Task.Handle<R> {
     let handle = Task.runDetached(operation: execute)

test/Concurrency/Runtime/future_fibonacci.swift

@@ -7,6 +7,12 @@
 
 import Dispatch
 
+#if canImport(Darwin)
+import Darwin
+#elseif canImport(Glibc)
+import Glibc
+#endif
+
 extension DispatchQueue {
   func async<R>(execute: @escaping () async throws -> R) -> Task.Handle<R> {
     let handle = Task.runDetached(operation: execute)