mtest: repeatedly run docker stop

[masa213f]

This PR fixes the following flaky test in the robustness test.

It's likely that docker stop kubelet failed. Therefore, I repeats docker stop until the Node becomes Not Ready.

https://github.com/cybozu-go/cke/actions/runs/21662518767/job/62450652922

Test CKE robustness should not update k8s components
/root/go/src/github.com/cybozu-go/cke/mtest/robustness_test.go:74
  STEP: stopping kubelet @ 02/04/26 07:39:29.946
  [FAILED] in [It] - /root/go/src/github.com/cybozu-go/cke/mtest/robustness_test.go:89 @ 02/04/26 07:42:30.045
• [FAILED] [180.098 seconds]
Test CKE robustness [It] should not update k8s components
/root/go/src/github.com/cybozu-go/cke/mtest/robustness_test.go:74

  [FAILED] Timed out after 180.002s.
  The function passed to Eventually failed at /root/go/src/github.com/cybozu-go/cke/mtest/robustness_test.go:88 with:
  Expected
      <v1.ConditionStatus>: True
  not to equal
      <v1.ConditionStatus>: True
  In [It] at: /root/go/src/github.com/cybozu-go/cke/mtest/robustness_test.go:89 @ 02/04/26 07:42:30.045

[YZ775]

LGTM