[CrOS Cellular] Fix flaky APN subpage close test

Speculative fix for flaky APN subpage test. The test below it is similar
but has await flushTasks() so this look like it may be necessary here.

Fixed: b:282998414
Test: *ApnSubpage*
Change-Id: I4295894d37a9ae58800c04eb9ce476ce5bd393ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4553356


Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Auto-Submit: Gordon Seto <gordonseto@google.com>
Reviewed-by: Regan Hsu <hsuregan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148007}