stdlib tests: add import statements to prevent unresolved symbols when compiling StdlibUnittest with -sil-serialize-all.

rdar://problem/18917405

test/1_stdlib/DictionaryUnchecked.swift

@@ -6,6 +6,13 @@
 
 import StdlibUnittest
 
+// Also import modules which are used by StdlibUnittest internally. This is
+// needed to link all required libraries in case we serialize StdlibUnittest.
+import SwiftPrivate
+#if _runtime(_ObjC)
+import ObjectiveC
+#endif
+
 var DictionaryUnchecked = TestSuite("DictionaryUnchecked")
 
 DictionaryUnchecked.test("noCseOnInit") {