Add tests/cppextdata data to TESTSUBDIRS

Author: pradyunsg

Makefile.pre.in

@@ -2029,7 +2029,7 @@ LIBSUBDIRS=	asyncio \
 		__phello__
 TESTSUBDIRS=	idlelib/idle_test \
 		test test/audiodata \
-		test/capath test/cjkencodings \
+		test/capath test/cppextdata test/cjkencodings \
 		test/data test/decimaltestdata \
 		test/dtracedata test/eintrdata \
 		test/encoded_modules test/imghdrdata \