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_prune_test.go

@@ -24,9 +24,7 @@ func TestContainersPruneError(t *testing.T) {
 	}
 
 	_, err := client.ContainersPrune(context.Background(), filters.Args{})
-	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 TestContainersPrune(t *testing.T) {