IDL: Add build target for IDL dictionary impl generation in core

Source/core/testing/TestingDictionary.idl

Index: Source/core/testing/TestingDictionary.idl
diff --git a/Source/core/streams/ReadableStream.idl b/Source/core/testing/TestingDictionary.idl
similarity index 50%
copy from Source/core/streams/ReadableStream.idl
copy to Source/core/testing/TestingDictionary.idl
index 927fa94b9a9604b286a95cbc8edbee1d4f3eb020..bf16e4b61fb6cf11a77953330877cdfbb9b17ee2 100644
--- a/Source/core/streams/ReadableStream.idl
+++ b/Source/core/testing/TestingDictionary.idl
@@ -3,7 +3,10 @@
 // found in the LICENSE file.
 
 [
-    RuntimeEnabled=Stream,
     GarbageCollected
-] interface ReadableStream {
+] dictionary TestingDictionary {
+    long longMember;
+    DOMString stringMember = "defaultStringValue";
+    boolean? booleanOrNullMember = null;
+    sequence<DOMString> stringSequenceMember;
 };