The test was using a static bucket name 'test-iam-bucket' that could conflict
with buckets created by other tests or previous runs. Each test framework
creates new RSA keys for JWT signing, so the 'admin-user' identity differs
between runs. When the bucket exists from a previous test, the new admin
cannot access or delete it, causing AccessDenied errors.
Changed to use GenerateUniqueBucketName() which ensures each test run gets
its own bucket, avoiding cross-test conflicts.
2763f105f4