Only use core dependencies for unit tests

[trexfeathers]

Paves the way for using testing to confirm that Iris' core code functions with only core dependencies.

• No optional dependency should be imported within iris/tests/unit files
• Existing calls to optional dependencies in iris/tests/unit should be refactored in one of two ways:
  • Highly granular unit testing: replace the calls with mocked calls.
  • More widely scoped tests: move to iris/tests/integration (re-writing them is out of scope here)
• iris/tests/integration should include at least one call down to each optional dependency