如何在保留测试可比性的同时使用动态错误？

In go I often use

func MyFunc(s someInterface) error {
    err := OtherFunc(s)
    return fmt.Errorf("something wrong: %s", err)
}

So I lose the original error value, because I just take the error string and forge it into a new error. That is what I mean with dynamic errors.

Now consider a test for MyFunc():

func TestMyFunc(t *testing.T) {
    s := mockSomeInterface()
    testErr := MyFunc(s)
    if testErr != interfaceSpecificErrorValue {
        t.Errorf("fail")
    }
}

What would I use for interfaceSpecificErrorValue (which is specific to someInterface in this example)? Or how could I make this testable?

I understand that I can solve this by defining all my possible errors beforehand and give them a constant value. What I am interested in is if there is another way to achieve this, because I like the hierarchical error messages that you can dynamically build up using fmt.Errorf("...: %s, err). There must be a good way to keep the error hierarchy without losing the original value.

(Comparing the output of the Error() method is a possibility, but not a good one.)

My preliminary answer to this is: In Go currently there is no canonical way to achieve nested errors with comparable error values.

After reading the offical documents and blog posts on error handling again, quite some source code from standard libraries, and finally this proposal: https://github.com/golang/proposal/blob/master/design/go2draft-error-values-overview.md, I decided I will keep my error types simple and static and wait for Go 2 to offer a better way.