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

Source/core/testing/DictionaryTest.idl

Index: Source/core/testing/DictionaryTest.idl
diff --git a/Source/core/streams/ReadableStream.idl b/Source/core/testing/DictionaryTest.idl
similarity index 61%
copy from Source/core/streams/ReadableStream.idl
copy to Source/core/testing/DictionaryTest.idl
index 927fa94b9a9604b286a95cbc8edbee1d4f3eb020..834dac4148b4b5129bb88d96e0cf7bd82e67053b 100644
--- a/Source/core/streams/ReadableStream.idl
+++ b/Source/core/testing/DictionaryTest.idl
@@ -3,7 +3,8 @@
 // found in the LICENSE file.
 
 [
-    RuntimeEnabled=Stream,
     GarbageCollected
-] interface ReadableStream {
+] interface DictionaryTest {
+    void set(optional TestingDictionary testingDictionary);
+    TestingDictionary get();
 };