fix(tests): Fix race condition causing flakiness in PHP generator test (

google#6213)

Turns out process.exit() was sometimes getting called before the
(async) fs.writeFile had finished writing the last of the generator
output files.

tests/generators/run_generators_in_browser.js

@@ -24,7 +24,7 @@ async function runLangGeneratorInBrowser(browser, filename, codegenFn) {
   await browser.execute(codegenFn);
   var elem = await browser.$("#importExport");
   var result = await elem.getValue();
-  fs.writeFile(filename, result, function(err) {
+  fs.writeFileSync(filename, result, function(err) {
     if (err) {
       return console.log(err);
     }