Change yarn test to default to experimental (#26741)

The idea is that the default `yarn test` command should be the one that
includes the most bleeding edge features, because during development you
probably want as many features enabled as possible.

That used to be `www-modern` but nowadays it's `experimental` because
we've landed a bunch of async actions stuff in experimental but it isn't
yet being tested at Meta.

So this switches the default to `experimental`.

scripts/jest/jest-cli.js

@@ -47,7 +47,7 @@ const argv = yargs
       describe: 'Run with the given release channel.',
       requiresArg: true,
       type: 'string',
-      default: 'www-modern',
+      default: 'experimental',
       choices: ['experimental', 'stable', 'www-classic', 'www-modern'],
     },
     env: {