puppeteer tests: Avoid stream creation flake.

We have a flake related to verifying that the app
prevents us from creating stream with duplicate names,
and my hypothesis is that it has to do with us not
waiting for the stream creation UI to fully appear. This flake
is probably a consequence of us recently making the stream
creation UI more like the stream editing UI, and thus
waiting for Desdemona to appear was giving us false
confidence that the page actually loaded.

I could be completely wrong about this solving the
flake, but the code change here is sensible regardless.

frontend_tests/puppeteer_tests/subscriptions.ts

@@ -81,6 +81,9 @@ async function test_subscription_button(page: Page): Promise<void> {
 
 async function click_create_new_stream(page: Page): Promise<void> {
     await page.click("#add_new_subscription .create_stream_button");
+    await page.waitForSelector(".finalize_create_stream", {visible: true});
+
+    // sanity check that desdemona is the initial subsscriber
     await await_user_visible(page, "desdemona");
 }