[iOS] Update field trial config and about_flags for CPE promo

This change:
* Updates the testing_field_trial_config.json and about_flags
  to add a new experiment/feature flag param. This param will allow
  all of the promo's triggers to be enabled at once.

* Checks for the CPE promo feature in the password manager tests.
  The addition to the fieldtrial config enables the copy password
  arm by default, which will cause the promo to appear in some
  tests.

This CL will be followed by an update to the CPE promo finch config.

Bug: None
Change-Id: I4c5b9a6f517f44ff0b977a401c1582f0cae27079
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4547450


Reviewed-by: Tommy Martino <tmartino@chromium.org>
Commit-Queue: Hira Mahmood <hiramahmood@google.com>
Cr-Commit-Position: refs/heads/main@{#1147895}