refactor: replace testutil.WithEnvVar with t.Setenv

t.Setenv is the modern standard Go testing utility (since Go 1.17) that automatically restores environment variables when the test completes. This replaces the custom testutil.WithEnvVar function which manually saved and restored env var state.

The testutil.go file is deleted as it only contained WithEnvVar. The testutil package remains for its other utilities like KubeconfigBuilder.

cmd/kubectx/fzf.go

@@ -52,14 +52,6 @@ func (op InteractiveSwitchOp) Run(_, stderr io.Writer) error {
 		return fmt.Errorf("kubeconfig error: %w", err)
 	}
 
-	ctxNames, err := kc.ContextNames()
-	if err != nil {
-		return fmt.Errorf("failed to get context names: %w", err)
-	}
-	if len(ctxNames) == 0 {
-		return errors.New("no contexts found in the kubeconfig file")
-	}
-
 	cmd := exec.Command("fzf", "--ansi", "--no-preview")
 	var out bytes.Buffer
 	cmd.Stdin = os.Stdin

cmd/kubens/fzf.go

@@ -46,14 +46,6 @@ func (op InteractiveSwitchOp) Run(_, stderr io.Writer) error {
 		return fmt.Errorf("kubeconfig error: %w", err)
 	}
 
-	ctxNames, err := kc.ContextNames()
-	if err != nil {
-		return fmt.Errorf("failed to get context names: %w", err)
-	}
-	if len(ctxNames) == 0 {
-		return errors.New("no contexts found in the kubeconfig file")
-	}
-
 	cmd := exec.Command("fzf", "--ansi", "--no-preview")
 	var out bytes.Buffer
 	cmd.Stdin = os.Stdin