Refactor test resources and create resource struct

[dmcgowan]

The test code should not be using a custom resource type which in turn must be created into the Resource types used by manifests.

r, err := Declare(Resource{
    Kind: RegularFile,
    Path: "/foo",
})

And for tests

[]Resource{
    MustDeclare(Resource{
        Kind: RegularFile,
        Path: "/foo",
    }),
}

Referenced from https://github.com/stevvooe/continuity/pull/14#discussion_r49527966