Prevent "testing" user from being on packet

[jabbate19]

Add some check when populating signatures_upper to prevent adding the user testing so that we don't have to keep deleting it.

[mxmeinhold]

We pull all the upperclassmen at a couple of points. One is when creating new packets, another is in sync_with_ldap. Both of these rely on get_active_members

There are a few cases where the testing user can be useful to have around, such as in dev or staging, but these aren't super important. The simplest solution I think would be to exclude the testing user from the ldap responses entirely. A slightly more complex solution that would still allow for testing would be to exclude the testing user only when deployed in production. This could be something handled by an environment/config variable, or it could be something we change in the actual image build. Lots of options!