Extend test to verify custom kubeconfigPath is used

cmd/kubeadm/app/cmd/init_test.go

@@ -17,7 +17,10 @@ limitations under the License.
 package cmd
 
 import (
+	"context"
 	"fmt"
+	"net/http"
+	"net/http/httptest"
 	"os"
 	"path/filepath"
 	"testing"
@@ -54,12 +57,12 @@ kind: ClusterConfiguration
 controlPlaneEndpoint: "3.4.5.6"
 `, kubeadmapiv1.SchemeGroupVersion.String(), expectedCRISocket)
 
-const testKubeconfigData = `---
+const testKubeconfigDataFormat = `---
 apiVersion: v1
 clusters:
 - name: foo-cluster
   cluster:
-    server: https://localhost:8080
+    server: %s
 contexts:
 - name: foo-context
   context:
@@ -365,8 +368,15 @@ func expectedInitIgnorePreflightErrors(expectedItems ...string) func(t *testing.
 }
 
 func TestInitDataClientWithNonDefaultKubeconfig(t *testing.T) {
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Method != http.MethodHead {
+			w.WriteHeader(http.StatusMethodNotAllowed)
+		}
+	}))
+	defer ts.Close()
+
 	kubeconfigPath := filepath.Join(t.TempDir(), "custom.conf")
-	if err := os.WriteFile(kubeconfigPath, []byte(testKubeconfigData), 0o600); err != nil {
+	if err := os.WriteFile(kubeconfigPath, []byte(fmt.Sprintf(testKubeconfigDataFormat, ts.URL)), 0o600); err != nil {
 		t.Fatalf("os.WriteFile returned unexpected error: %v", err)
 	}
 
@@ -381,8 +391,13 @@ func TestInitDataClientWithNonDefaultKubeconfig(t *testing.T) {
 		t.Fatalf("newInitData returned unexpected error: %v", err)
 	}
 
-	// This only works if the client that is constructed from the kubeconfig is not used.
-	if _, err := data.Client(); err != nil {
+	client, err := data.Client()
+	if err != nil {
 		t.Fatalf("data.Client returned unexpected error: %v", err)
 	}
+
+	result := client.Discovery().RESTClient().Verb("HEAD").Do(context.Background())
+	if err := result.Error(); err != nil {
+		t.Fatalf("REST client request returned unexpected error: %v", err)
+	}
 }