Removing short timeout on ContainerExecDecoratorTest.testContainerExecPerformance

[jglick]

Amends #427, since in a build from #503 I saw an apparently unrelated flake

org.junit.runners.model.TestTimedOutException: test timed out after 10000 milliseconds
	at sun.misc.Unsafe.park(Native Method)
	at java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:215)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireSharedNanos(AbstractQueuedSynchronizer.java:1037)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.tryAcquireSharedNanos(AbstractQueuedSynchronizer.java:1328)
	at java.util.concurrent.CountDownLatch.await(CountDownLatch.java:277)
	at io.fabric8.kubernetes.client.internal.readiness.ReadinessWatcher.await(ReadinessWatcher.java:58)
	at io.fabric8.kubernetes.client.dsl.base.HasMetadataOperation.periodicWatchUntilReady(HasMetadataOperation.java:192)
	at io.fabric8.kubernetes.client.dsl.base.HasMetadataOperation.waitUntilReady(HasMetadataOperation.java:217)
	at io.fabric8.kubernetes.client.dsl.base.HasMetadataOperation.waitUntilReady(HasMetadataOperation.java:36)
	at org.csanchez.jenkins.plugins.kubernetes.pipeline.ContainerExecDecorator$1.waitUntilPodContainersAreReady(ContainerExecDecorator.java:479)
	at org.csanchez.jenkins.plugins.kubernetes.pipeline.ContainerExecDecorator$1.doLaunch(ContainerExecDecorator.java:292)
	at org.csanchez.jenkins.plugins.kubernetes.pipeline.ContainerExecDecorator$1.launch(ContainerExecDecorator.java:273)
	at org.csanchez.jenkins.plugins.kubernetes.pipeline.ContainerExecDecoratorTest.execCommand(ContainerExecDecoratorTest.java:325)
	at org.csanchez.jenkins.plugins.kubernetes.pipeline.ContainerExecDecoratorTest.testContainerExecPerformance(ContainerExecDecoratorTest.java:312)

with logs

…
0.021 [id=741]	FINEST	o.c.j.p.k.p.ContainerExecDecorator$1#waitUntilPodContainersAreReady: Waiting until pod containers are ready: kubernetes-plugin-test/test-command-execution-…
10.035 [id=1]	WARNING	o.c.j.p.k.KubernetesTestUtil#deletePods: Deleting leftover pods: [test-command-execution-… (Pending)]

consistent with the busybox image not having been cached at the time the test runs, for example.