Fix the flaky IpcDesktopEnvironment unit tests

3 flaky tests were identified after I landed my recent IPC->Mojo
conversion CL.  After investigating, it looks like there is a race
condition in the unit tests were the UrlForwarderConfigurator was
being called before the desktop was attached.

The timing of when the desktop is actually attached did change in
my CL but callers can reliably wait until they receive the attached
event to know when it is ready.  The flaky tests would call
IsUrlForwarderSetUp() and then wait for the attached event. This
appears to have worked fine in the past but it's no longer reliable
so I fixed the tests by calling IsUrlForwarderSetUp() after we know
the desktop has been attached.

Bug: 1291438, 1291487, 1291605
Change-Id: I9bb0e1e394be30e3895a6dab618c4b4350658a1a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3425183
Auto-Submit: Joe Downing <joedow@chromium.org>
Reviewed-by: Yuwei Huang <yuweih@chromium.org>
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Cr-Commit-Position: refs/heads/main@{#964734}