maint: Refactor failing tests

When testing our custom matcher we want to ensure they also fail a test
appropiatly (e.g. you test a mock was never called, but it was indeed
called; we want to know this fails)

Previously we did write a test like a user would write the test but
instead of using `it` we use `if.fails`.

While this did work, we are unable to validate *why* the test fails. We
now don't assume failing tests but instead we wrap these in
`expect(...).toThrow`.

This should not only be easier to read but also allows us to capture the
error message to validate the test fails for the correct reason.