Change test_mode to an app property

[timrid]

In this PR, the test_mode Flag becomes a property of the AppConfig as suggested in #2173 (review). This simplifies the code, as test_mode is only required at a few points and no longer has to be passed through all functions.

This refactoring simplifies adding a new debug_mode in #2173.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

It's a big change, but I think the shape of the changes that have been required in the test suite indicates it's the right move - it's mostly removing test_mode context from places where it isn't really relevant, so we can inject it explicitly in the handful of places where it matters.

Thanks for taking this on - most appreciated!