Add small sleep to retryment for GoneError

[daipom]

• Fixes Stabilize test cases which depends on execution timing #408

Fix unstable tests:

TestWatchNamespaces#test_0012_namespace watch retries when 410 Gone errors are encountered:
NoMethodError: undefined method `>=' for nil:NilClass
    test/plugin/test_watch_namespaces.rb:250:in `block (3 levels) in <class:TestWatchNamespaces>'
    test/plugin/test_watch_namespaces.rb:241:in `block (2 levels) in <class:TestWatchNamespaces>'
    test/plugin/test_watch_namespaces.rb:240:in `block in <class:TestWatchNamespaces>'

TestWatchPods#test_0013_pod watch retries when 410 Gone errors are encountered:
NoMethodError: undefined method '>=' for nil
    test/plugin/test_watch_pods.rb:360:in 'block (3 levels) in <class:TestWatchPods>'
    test/plugin/test_watch_pods.rb:351:in 'block (2 levels) in <class:TestWatchPods>'
    test/plugin/test_watch_pods.rb:350:in 'block in <class:TestWatchPods>'

It appears that the cause could be the infinite loop without any sleep when GoneError occurs.