move Backend interface to backend package

internal/backend/backend_test.go

@@ -0,0 +1,38 @@
+package backend_test
+
+import (
+	"testing"
+
+	"github.com/restic/restic/internal/backend"
+	"github.com/restic/restic/internal/test"
+)
+
+type testBackend struct {
+	backend.Backend
+}
+
+func (t *testBackend) Unwrap() backend.Backend {
+	return nil
+}
+
+type otherTestBackend struct {
+	backend.Backend
+}
+
+func (t *otherTestBackend) Unwrap() backend.Backend {
+	return t.Backend
+}
+
+func TestAsBackend(t *testing.T) {
+	other := otherTestBackend{}
+	test.Assert(t, backend.AsBackend[*testBackend](other) == nil, "otherTestBackend is not a testBackend backend")
+
+	testBe := &testBackend{}
+	test.Assert(t, backend.AsBackend[*testBackend](testBe) == testBe, "testBackend was not returned")
+
+	wrapper := &otherTestBackend{Backend: testBe}
+	test.Assert(t, backend.AsBackend[*testBackend](wrapper) == testBe, "failed to unwrap testBackend backend")
+
+	wrapper.Backend = other
+	test.Assert(t, backend.AsBackend[*testBackend](wrapper) == nil, "a wrapped otherTestBackend is not a testBackend")
+}