client: update error-assertions in tests

- use is.ErrorType
- replace uses of client.IsErrNotFound for errdefs.IsNotFound, as
  the client no longer returns the old error-type.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

client/container_restart_test.go

@@ -11,6 +11,8 @@ import (
 
 	"github.com/docker/docker/api/types/container"
 	"github.com/docker/docker/errdefs"
+	"gotest.tools/v3/assert"
+	is "gotest.tools/v3/assert/cmp"
 )
 
 func TestContainerRestartError(t *testing.T) {
@@ -18,9 +20,7 @@ func TestContainerRestartError(t *testing.T) {
 		client: newMockClient(errorMock(http.StatusInternalServerError, "Server error")),
 	}
 	err := client.ContainerRestart(context.Background(), "nothing", container.StopOptions{})
-	if !errdefs.IsSystem(err) {
-		t.Fatalf("expected a Server Error, got %[1]T: %[1]v", err)
-	}
+	assert.Check(t, is.ErrorType(err, errdefs.IsSystem))
 }
 
 func TestContainerRestart(t *testing.T) {