backend/cache: use backend types not restic types (#21860)

internal/backend/cache/testing.go

@@ -3,16 +3,17 @@ package cache
 import (
 	"testing"
 
-	"github.com/restic/restic/internal/restic"
 	"github.com/restic/restic/internal/test"
 )
 
+const testCacheID = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+
 // TestNewCache returns a cache in a temporary directory which is removed when
 // cleanup is called.
 func TestNewCache(t testing.TB) *Cache {
 	dir := test.TempDir(t)
 	t.Logf("created new cache at %v", dir)
-	cache, err := New(restic.NewRandomID().String(), dir)
+	cache, err := New(testCacheID, dir)
 	if err != nil {
 		t.Fatal(err)
 	}