[iOS] Disable UserPolicyTestCase tests

https://ci.chromium.org/ui/p/chrome/builders/ci/iphone-device/43216/overview

Following tests are disabled:

- testSigninFlowConfirmationDialogNotShownWhenAlreadyBrowserPolicies
- testSigninFlowConfirmationDialogWhenUserPolicyAndSignin
- testThatPoliciesAreClearedOnSignOut
- testThatPoliciesAreFetchedOnSignIn
- testThatPoliciesAreLoadedFromStoreWhenSignedInAtStartup

Bug: 331794057
Change-Id: I24273eb56b7919c7155d58ad5179617e6f19d735
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5406840


Commit-Queue: Ginny Huang <ginnyhuang@chromium.org>
Owners-Override: Ginny Huang <ginnyhuang@chromium.org>
Commit-Queue: Hira Mahmood <hiramahmood@google.com>
Reviewed-by: Hira Mahmood <hiramahmood@google.com>
Auto-Submit: Ginny Huang <ginnyhuang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280211}