Make tests import new Synchronization module

Author: Azoy

test/stdlib/Atomics/AtomicLazyReference.swift

@@ -1,6 +1,7 @@
 // RUN: %target-run-simple-swift
 // REQUIRES: executable_test
 
+import Synchronization
 import StdlibUnittest
 
 let suite = TestSuite("AtomicLazyReference")

test/stdlib/Atomics/Basics/Tests.gyb-template

@@ -7,6 +7,7 @@
 /// Exercises all operations in a single-threaded context, verifying
 /// they provide the expected results.
 
+import Synchronization
 import StdlibUnittest
 
 % for label, type, a, b in types:

test/stdlib/Atomics/LockFreeSingleConsumerStack.swift

@@ -2,6 +2,7 @@
 // REQUIRES: executable_test
 // REQUIRES: libdispatch
 
+import Synchronization
 import StdlibUnittest
 import Dispatch
 

test/stdlib/Atomics/WordPair.swift

@@ -1,6 +1,7 @@
 // RUN: %target-run-simple-swift
 // REQUIRES: executable_test
 
+import Synchronization
 import StdlibUnittest
 
 let suite = TestSuite("WordPairTests")