Ensure that json extension is enabled when running tests.

The `commonjs_modules.phpt` test requires `json_encode` in order to
complete successfully.

Makefile.frag

@@ -1,3 +1,4 @@
+PHP_TEST_SHARED_EXTENSIONS+=-d extension=$(EXTENSION_DIR)/json.so
 
 testv8: all
 	$(PHP_EXECUTABLE) -n -d extension_dir=./modules -d extension=v8js.so test.php