fixup : try --output-filename instead of -o

-o does not work, the script still outputs to
`./dist/browser-polyfill.js`.

scripts/run-module-bundlers-smoketests.js

@@ -8,7 +8,7 @@ Test webextension-polyfill bundled with webpack
 ===============================================`);
 
 process.env.TEST_BUNDLED_POLYFILL = "/tmp/webpack-bundle.js";
-result = shell.exec(`webpack --mode production --entry ./test/fixtures/bundle-entrypoint.js -o ${process.env.TEST_BUNDLED_POLYFILL}`).code ||
+result = shell.exec(`webpack --mode production --entry ./test/fixtures/bundle-entrypoint.js --output-filename ${process.env.TEST_BUNDLED_POLYFILL}`).code ||
   shell.exec("npm run test").code || result;
 
 console.log(`