[CrOS Settings] Fix flaky CellularNetworksList test failures.

Fix flaky OSSettingsCellularNetworksListV3Test failures caused by DOM
not rendering completely before testing started. This is fixed by
triggering the DOM to flush again before executing the rest of the test.
This fix is confirmed by running the test with [100/100] passes.

Fixed: 1164157
Change-Id: Ic014252a5ed749bc6641e6a6ab2f21383f09b165
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2618480Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Gordon Seto <gordonseto@google.com>
Cr-Commit-Position: refs/heads/master@{#841602}

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

@@ -166,7 +166,8 @@ suite('CellularNetworkList', function() {
 
     let eSimNetworkList = cellularNetworkList.$$('#esimNetworkList');
     assertTrue(!!eSimNetworkList);
-    assertEquals(1, cellularNetworkList.eSimPendingProfileItems_.length);
+
+    Polymer.dom.flush();
 
     const listItem = eSimNetworkList.$$('network-list-item');
     assertTrue(!!listItem);