test: tmpdir can be a test helper

Signed-off-by: Justin Chadwell <me@jedevc.com>

tests/build.go

@@ -148,11 +148,10 @@ RUN cp /etc/foo /etc/bar
 FROM scratch
 COPY --from=base /etc/bar /bar
 `)
-	dir, err := tmpdir(
+	dir := tmpdir(
 		t,
 		fstest.CreateFile("Dockerfile", dockerfile, 0600),
 		fstest.CreateFile("foo", []byte("foo"), 0600),
 	)
-	require.NoError(t, err)
 	return dir
 }

tests/integration.go

@@ -7,14 +7,15 @@ import (
 
 	"github.com/containerd/continuity/fs/fstest"
 	"github.com/moby/buildkit/util/testutil/integration"
+	"github.com/stretchr/testify/require"
 )
 
-func tmpdir(t *testing.T, appliers ...fstest.Applier) (string, error) {
+func tmpdir(t *testing.T, appliers ...fstest.Applier) string {
+	t.Helper()
 	tmpdir := t.TempDir()
-	if err := fstest.Apply(appliers...).Apply(tmpdir); err != nil {
-		return "", err
-	}
-	return tmpdir, nil
+	err := fstest.Apply(appliers...).Apply(tmpdir)
+	require.NoError(t, err)
+	return tmpdir
 }
 
 func buildxCmd(sb integration.Sandbox, args ...string) *exec.Cmd {