Improve usage of mocks in testing

[jemrobinson]

There are several ways in which we could improve our use of mocks:

1. Checking for asserts

assert "mock deploy" in result.stdout
assert "mock deploy error" in result.stdout

I think I used this kind of pattern before I realised Mock.assert_called() and related methods existed.

Originally posted by @JimMadge in #1982 (comment)

2. Better consolidation of default config files/objects

3. Clarity about the best way to mock objects with complex __init__s