Fix flaky CanManageWallet

Clicking on Sign Transaction in the Wallet Send page, will, when a hot
wallet is setup, use PostRedirect page to redirect to the
broadcast screen. The problem was that sometimes, s.Driver.PageSource
would return this PostRedirect page rather than the broadcast page.
Waiting for an element of the broadcast page fixes this issue.

BTCPayServer.Tests/SeleniumTests.cs

@@ -1973,6 +1973,7 @@ namespace BTCPayServer.Tests
             var jack = new Key().PubKey.Hash.GetAddress(Network.RegTest);
             SetTransactionOutput(s, 0, jack, 0.01m);
             s.Driver.FindElement(By.Id("SignTransaction")).Click();
+            s.Driver.WaitForElement(By.CssSelector("button[value=broadcast]"));
             Assert.Contains(jack.ToString(), s.Driver.PageSource);
             Assert.Contains("0.01000000", s.Driver.PageSource);
             Assert.EndsWith("psbt/ready", s.Driver.Url);