Close auth clients in tctl tests (#24014)

Goroutine dumps from failed tests show a large number of goroutines
blocked in GRPC client code. Our tctl tests create auth clients but
only expose them via auth.ClientI, which doesn't have a close method.

Updates #19372 and #24005

tool/tctl/common/helpers_test.go

@@ -83,6 +83,13 @@ func getAuthClient(ctx context.Context, t *testing.T, fc *config.FileConfig, opt
 
 	client, err := authclient.Connect(ctx, clientConfig)
 	require.NoError(t, err)
+
+	t.Cleanup(func() {
+		if closer, ok := client.(io.Closer); ok {
+			closer.Close()
+		}
+	})
+
 	return client
 }