Improve error checks in tests

[dbanck]

Go 1.19 discovered a minor flaw in our tests in #129: second argument to errors.As should not be *error

Changing the type of expectedErr in our test table to interface{} silenced that error for now but defeated the purpose of testing if the function throws the correct error. So currently, any error (or even any non nil value) will make the test pass.

Proposal

There are a couple of different solutions to solve this:

1. compare string representations, i.e. given.Error() == expected.Error()
2. use reflection
3. avoid table tests altogether
4. include a pointer to a function which takes an error and returns a boolean