internal/stub: move from container

This package solves a very specific stubbing use case, in a less than elegant manner.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/stub/errors.go

@@ -0,0 +1,28 @@
+package stub
+
+import (
+	"errors"
+	"strconv"
+)
+
+var (
+	ErrCheck = errors.New("one or more arguments did not match")
+)
+
+// UniqueError is an error that only equivalates to another [UniqueError] with
+// the same magic value.
+type UniqueError uintptr
+
+func (e UniqueError) Error() string {
+	return "unique error " +
+		strconv.FormatUint(uint64(e), 10) +
+		" injected by the test suite"
+}
+
+func (e UniqueError) Is(target error) bool {
+	var u UniqueError
+	if !errors.As(target, &u) {
+		return false
+	}
+	return e == u
+}