Automatic merge from submit-queue

Fix overlong junit filename prefixes (2nd attempt)

This is followup for #30894.
Turned out that filename shortening I used isn't enough in some cases (scroll to the right):
```
vagrant@devbox:~/work/kubernetes/src/k8s.io/kubernetes (test %) $ mkdir -p /tmp/art && time KUBE_JUNIT_REPORT_DIR=/tmp/art KUBE_COVER=y make test WHAT=cmd/libs/go2idl/client-gen/testoutput/clientset_generated/test_internalclientset/typed/testgroup.k8s.io/unversioned
Running tests for APIVersion: v1,apps/v1alpha1,authentication.k8s.io/v1beta1,authorization.k8s.io/v1beta1,autoscaling/v1,batch/v1,batch/v2alpha1,certificates/v1alpha1,extensions/v1beta1,federation/v1beta1,policy/v1alpha1,rbac.authorization.k8s.io/v1alpha1,imagepolicy.k8s.io/v1alpha1
+++ [0822 13:57:46] Saving coverage output in '/tmp/k8s_coverage/v1,apps/v1alpha1,authentication.k8s.io/v1beta1,authorization.k8s.io/v1beta1,autoscaling/v1,batch/v1,batch/v2alpha1,certificates/v1alpha1,extensions/v1beta1,federation/v1beta1,policy/v1alpha1,rbac.authorization.k8s.io/v1alpha1,imagepolicy.k8s.io/v1alpha1/20160822-135746'
tee: /tmp/art/junit_v1,apps-v1a1,authentication.v1b1,authorization.v1b1,autoscaling-v1,batch-v1,batch-v2a1,certificates-v1a1,extensions-v1b1,federation-v1b1,policy-v1a1,rbac.authorization.v1a1,imagepolicy.v1a1_20160822-135746-cmd_libs_go2idl_client-gen_testoutput_clientset_generated_test_internalclientset_typed_testgroup.k8s.io_unversioned.stdout: File name too long
ok      k8s.io/kubernetes/cmd/libs/go2idl/client-gen/testoutput/clientset_generated/test_internalclientset/typed/testgroup.k8s.io/unversioned   0.038s
ls: cannot access '/tmp/art/junit_v1,apps-v1a1,authentication.v1b1,authorization.v1b1,autoscaling-v1,batch-v1,batch-v2a1,certificates-v1a1,extensions-v1b1,federation-v1b1,policy-v1a1,rbac.authorization.v1a1,imagepolicy.v1a1_20160822-135746*.stdout': No such file or directory
Makefile:118: recipe for target 'test' failed
make: *** [test] Error 1

real    0m49.623s
user    2m35.224s
sys     0m9.200s
```

Looks like we're have no choice here besides just using a hash as filename prefix.