This change adds two new test assertion helpers, `AssertErrorIs`
and `AssertErrorWraps`. The former is a wrapper around `errors.Is`,
and asserts that the error's wrapping chain contains a specific (i.e.
singleton) error. The latter is a wrapper around `errors.As`, and
asserts that the error's wrapping chain contains any error which is
of the given type; it also has the same unwrapping side effect as
`errors.As`, which can be useful for further assertions about the
contents of the error.
It also makes two small changes to our `berrors` package, namely
making `berrors.ErrorType` itself an error rather than just an int,
and giving `berrors.BoulderError` an `Unwrap()` method which
exposes that inner `ErrorType`. This allows us to use the two new
helpers above to make assertions about berrors, rather than
having to hand-roll equality assertions about their types.
Finally, it takes advantage of the two changes above to greatly
simplify many of the assertions in our tests, removing conditional
checks and replacing them with simple assertions.
294d1c31d7