[Profile picker] Disable a flaky test

The test CreateSignedInProfileSigninAlreadyExists is flaky on multiple
platforms.

Bug: 1144065
Change-Id: I6382c9d8757de99c1e5575a1101bdae647dc0cdc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521184


Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Commit-Queue: David Roger <droger@chromium.org>
Auto-Submit: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824825}