Make less strict ESLint config for testing files

[Siegrift]

Testing requires creating mocks and using fixtures and sometimes uglier mutable code. This is fine (and sometimes preferred) for tests, but with our strict production ESLint configuration we need to use multiple eslint disable comments which is ugly.

We could turn off the problematic rules in the test config (only applied for tests files based on filename pattern).

[dcroote]

Close now that the eslint plugin has moved?

[Siegrift]

I'd transfer instead. That said, this is not necessarily the priority.