update file mock to return file name

__mocks__/fileMock.js

@@ -1 +1,7 @@
-module.exports = 'test-file-stub';
+const path = require('path');
+
+module.exports = {
+  process (src, filename) {
+    return `module.exports=${JSON.stringify(path.basename(filename))};`;
+  },
+};