move custom assertion to test-helper.js (`isPlainObjectOrFunc` & `isProperSubsetOf`)
let's call the custom assertion `isProperSubset` (without the "Of" in the end) as alleged *subset* comes right after it when using assert syntax; for the expectation suffix "Of" does make sense though
remove (some) lints
start writing tests for custom assertions `isPlainObjectOrFunc` and `isProperSubset`