Added TestFuture in WebKioskAppLauncherTest,

WebKioskAppServiceLauncherTest

Replacing the usage of base::RunLoop with base::test::TestFuture for better code quality.

Bug: b/255714505
Test: Ran the tests involved.
Change-Id: Ie6913d34945eed9a3b053778a3988ff567d6e9c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3998660
Reviewed-by: Ben Franz <bfranz@chromium.org>
Reviewed-by: Jeroen Dhollander <jeroendh@google.com>
Commit-Queue: Ashutosh Singhal <macinashutosh@google.com>
Cr-Commit-Position: refs/heads/main@{#1067488}