The Test method was only used in exactly one place, namely when trying
to create a new repository it was used to check whether a config file
already exists.
Use a combination of Stat() and IsNotExist() instead.
The ioutil functions are deprecated since Go 1.17 and only wrap another
library function. Thus directly call the underlying function.
This commit only mechanically replaces the function calls.
The RetryBackend tests depend on the mock backend. When the Backend
interface is eventually split from the restic package, this will lead to
a dependency cycle between backend and backend/mock. Thus split the
RetryBackend into a separate package to avoid this problem.
