backend: Adjust tests to use the Factory to instantiate the backend

This drastically reduces the amount of duplicated test code.

internal/backend/mem/mem_backend_test.go

@@ -1,58 +1,20 @@
 package mem_test
 
 import (
-	"context"
 	"testing"
 
-	"github.com/restic/restic/internal/errors"
-	"github.com/restic/restic/internal/restic"
-
 	"github.com/restic/restic/internal/backend/mem"
 	"github.com/restic/restic/internal/backend/test"
 )
 
-type memConfig struct {
-	be restic.Backend
-}
-
-func newTestSuite() *test.Suite[*memConfig] {
-	return &test.Suite[*memConfig]{
+func newTestSuite() *test.Suite[struct{}] {
+	return &test.Suite[struct{}]{
 		// NewConfig returns a config for a new temporary backend that will be used in tests.
-		NewConfig: func() (**memConfig, error) {
-			cfg := &memConfig{}
-			return &cfg, nil
+		NewConfig: func() (*struct{}, error) {
+			return &struct{}{}, nil
 		},
 
-		// CreateFn is a function that creates a temporary repository for the tests.
-		Create: func(cfg *memConfig) (restic.Backend, error) {
-			if cfg.be != nil {
-				_, err := cfg.be.Stat(context.TODO(), restic.Handle{Type: restic.ConfigFile})
-				if err != nil && !cfg.be.IsNotExist(err) {
-					return nil, err
-				}
-
-				if err == nil {
-					return nil, errors.New("config already exists")
-				}
-			}
-
-			cfg.be = mem.New()
-			return cfg.be, nil
-		},
-
-		// OpenFn is a function that opens a previously created temporary repository.
-		Open: func(cfg *memConfig) (restic.Backend, error) {
-			if cfg.be == nil {
-				cfg.be = mem.New()
-			}
-			return cfg.be, nil
-		},
-
-		// CleanupFn removes data created during the tests.
-		Cleanup: func(cfg *memConfig) error {
-			// no cleanup needed
-			return nil
-		},
+		Factory: mem.NewFactory(),
 	}
 }