Stabilize flaky remoteStorage Redis spec

"stores a token in redis" asserted an exact count of Redis keys for the
user, but database cleanup only covers SQL and Redis keys accumulate
across examples and spec files. It also relied on a previously-run
example having created the authorization, so it failed depending on test
order and leftover state.

Clear the authorizations:* namespace before each example in the #create
group, create the authorization under test, and assert on its specific
token key instead of a global count.

spec/models/remote_storage_authorization_spec.rb

@@ -11,6 +11,7 @@ RSpec.describe RemoteStorageAuthorization, type: :model do
   end
 
   describe "#create" do
+    before(:each) { redis_rs_delete_keys("authorizations:*") }
     after(:each) { clear_enqueued_jobs }
     after(:all)  { redis_rs_delete_keys("authorizations:*") }
 
@@ -27,10 +28,9 @@ RSpec.describe RemoteStorageAuthorization, type: :model do
     end
 
     it "stores a token in redis" do
-      user_auth_keys = redis_rs.keys("authorizations:#{user.cn}:*")
-      expect(user_auth_keys.length).to eq(1)
+      auth
 
-      authorizations = redis_rs.smembers(user_auth_keys.first)
+      authorizations = redis_rs.smembers("authorizations:#{user.cn}:#{auth.token}")
       expect(authorizations.sort).to eq(%w(documents photos contacts:rw videos:r tasks/work:r).sort)
     end