Make gentest headless by default

Summary:
This makes `yarn gentest` a little less annoying. It seems a lot slower/less rliable than it used to be, for reasons I don't understand, but really could use a rejiggering.

https://developer.chrome.com/docs/chromium/headless

{F1985838453}

Differential Revision: D93960790

Author: NickGerleman

.github/workflows/validate-tests.yml

@@ -21,7 +21,7 @@ jobs:
         run: yarn gentest-validate
 
       - name: yarn gentest
-        run: yarn gentest -h
+        run: yarn gentest
 
       - name: Check for modified tests
         run: |

gentest/gentest-driver.ts

@@ -31,7 +31,7 @@ function addSignatureToSourceCode(sourceCode: string): string {
 const argv = minimist(process.argv.slice(2));
 const specificFixture = argv.f || argv.fixture;
 const suspend = argv.s || argv.suspend;
-const headless = argv.h || argv.headless;
+const createWindow = argv.w || argv['create-window'];
 
 const gentestDir = dirname(fileURLToPath(import.meta.url));
 const yogaDir = dirname(gentestDir);
@@ -55,7 +55,9 @@ options.addArguments(
   '--window-position=0,0',
   '--hide-scrollbars',
 );
-headless && options.addArguments('--headless');
+if (!createWindow) {
+  options.addArguments('--headless');
+}
 options.setLoggingPrefs({
   browser: 'ALL',
   performance: 'ALL',