Fix failing tests

Fix the gsutil fetch tests broken by the gcloud storage migration and
the Firefox integration tests broken by pruned archives.

gsutil fetch tests:

The automated migration in 833992f switched GsUtilDownloader from
`gsutil.py cp` to `gcloud storage cp` but left
test/fetch_gsutil_test.dart untouched, breaking all three of its
tests.

* The two mocked tests still passed 'gsutil.py' as the expected
  executable. They now expect 'gcloud' and also assert the full
  argument list, whose absence is what let the migration slip
  through in the first place.
* 'fetch gsutil package' actually ran the executable and relied on
  gsutil accepting a local-to-local copy. `gcloud storage cp`
  rejects that with "Local copies not supported", so the test now
  mocks the process and performs the copy as a side effect, keeping
  the assertions on the value returned by fetch().
* Drop the '401 Anonymous caller' special case and its test. That
  wording was gsutil's. `gcloud storage cp` reports an inaccessible
  object as "not found: 404" and, when credentials are missing,
  already prints "Please run: $ gcloud auth login" itself, so the
  hand-rolled StateError was both unreachable and redundant with
  the stderr carried by the ProcessException.

Firefox integration tests:

The Firefox 135.0 build2 archives have been pruned from
archive.mozilla.org and now return 404. Bump the integration tests to
157.0 build1, the current release.

Bug: b/486536908
Change-Id: I777c7a998b96ad14f849c31b189b90224da06de4
TAG=agy
CONV=13b61641-faa1-4873-a042-3cee827fa991
Reviewed-on: https://dart-review.googlesource.com/c/deps/+/550220
Auto-Submit: Alexander Thomas <athom@google.com>
Reviewed-by: Ivan Inozemtsev <iinozemtsev@google.com>
Commit-Queue: Ivan Inozemtsev <iinozemtsev@google.com>