tests: rename "kubectl" setupTestNamespace

As the other version has a different signature, but exists for a
different build tag. Resulting in my IDE becoming absolutely confused
when I tried to enable both at the same time. Opted for "exec" because
this one shells out.

Signed-off-by: Hidde Beydals <hello@hidde.co>

cmd/flux/main_e2e_test.go

@@ -65,7 +65,7 @@ func TestMain(m *testing.M) {
 	os.Exit(code)
 }
 
-func setupTestNamespace(namespace string) (func(), error) {
+func execSetupTestNamespace(namespace string) (func(), error) {
 	kubectlArgs := []string{"create", "namespace", namespace}
 	_, err := utils.ExecKubectlCommand(context.TODO(), utils.ModeStderrOS, *kubeconfigArgs.KubeConfig, *kubeconfigArgs.Context, kubectlArgs...)
 	if err != nil {