GUILD-619: Test helpers don't automatically reset to test config

Description

Deimos is not reset back to the default test config before each test now.
A function has been added which can be called by clients in tests to manually reset Deimos back to test config.
The same function can also be used to override settings while keeping defaults in place.

[ ] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[x] This change requires a documentation update

Tested by calling this function in existing specs locally.

[x] My code follows the style guidelines of this project
[x] I have performed a self-review of my own code
[x] I have commented my code, particularly in hard-to-understand areas
[x] I have made corresponding changes to the documentation
[x] I have added a line in the CHANGELOG describing this change, under the UNRELEASED heading
[x] My changes generate no new warnings
[ ] I have added tests that prove my fix is effective or that my feature works
[x] New and existing unit tests pass locally with my changes
[ ] Any dependent changes have been merged and published in downstream modules