Test_cv_subscribe_system

Third time the charm
I think that this class can — and should — be dropped entirely. Here's my reasoning:

1. A search facility, if added, should be general-purpose. For some entities, I want to search by name and organization. But for other entities, I want to search by other parameters. For example, what if I want to search for a particular organization? Specifying an organization ID doesn't make sense in that case.
2. We don't yet have a good, general-case knowledge of how searching will be used. Instead, performing searches is something that's currently done in a few, very specific contexts. For example, when creating a new lifecycle environment, a search must be performed. But right now, that's the one and only example of a search *really* being needed for testing.
3. Searching is a feature that is independent of creating. Some entities can not created, but we may wish to search for them anyway. So an `EntitySearchMixin`, if created, should be independent of `EntityFactoryMixin`.

Some specific things I don't like about this code:

1. When searching, the `params` argument to `client.get` should be specified, not the `data` parameter.
2. Calling `attributes()` generates random attributes. So we're searching using random query parameters. That's nonsensical.

p.s. @AdamSaleh, please don't take this criticism personally. It's about the code, not you.