[CrOS Network] Fix flaky tether dialog test.

Fix flaky "Create tether network, first connection attempt shows tether
dialog" test by making test wait for dialog to be opened first. This fix
is speculative as the test flakiness could not be reproduced locally.

(cherry picked from commit d4c210d)

Fixed: b/263411073
Test: *InternetDetailPage*
Change-Id: I796f1e6af00701f850c147e5b1a5c2754b81a5a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4128025
Auto-Submit: Gordon Seto <gordonseto@google.com>
Reviewed-by: Theo Johnson-kanu <tjohnsonkanu@google.com>
Commit-Queue: Gordon Seto <gordonseto@google.com>
Cr-Original-Commit-Position: refs/heads/main@{#1087453}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4204949
Commit-Queue: Theo Johnson-kanu <tjohnsonkanu@google.com>
Cr-Commit-Position: refs/branch-heads/5481@{#809}
Cr-Branched-From: 130f3e4-refs/heads/main@{#1084008}

chrome/test/data/webui/settings/chromeos/internet_detail_page_tests.js

@@ -1525,8 +1525,18 @@ suite('InternetDetailPage', function() {
           assertTrue(!!tetherDialog);
           assertFalse(tetherDialog.$.dialog.open);
 
+          let showTetherDialogFinished;
+          const showTetherDialogPromise = new Promise((resolve) => {
+            showTetherDialogFinished = resolve;
+          });
+          const showTetherDialog = internetDetailPage.showTetherDialog_;
+          internetDetailPage.showTetherDialog_ = () => {
+            showTetherDialog.call(internetDetailPage);
+            showTetherDialogFinished();
+          };
+
           connect.click();
-          await flushAsync();
+          await showTetherDialogPromise;
           assertTrue(tetherDialog.$.dialog.open);
         });