Move kubeconfig loader utils to cmdutil pkg

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

internal/testutil/tempfile.go

@@ -0,0 +1,26 @@
+package testutil
+
+import (
+	"io/ioutil"
+	"os"
+	"testing"
+)
+
+func TempFile(t *testing.T, contents string) (path string, cleanup func()) {
+	// TODO consider removing, used only in one place.
+	t.Helper()
+
+	f, err := ioutil.TempFile(os.TempDir(), "test-file")
+	if err != nil {
+		t.Fatalf("failed to create test file: %v", err)
+	}
+	path = f.Name()
+	if _, err := f.Write([]byte(contents)); err != nil {
+		t.Fatalf("failed to write to test file: %v", err)
+	}
+
+	return path, func() {
+		f.Close()
+		os.Remove(path)
+	}
+}