[feature] [cli] ability to specify multiple -focus and -skip flags

[kolyshkin]

I would like to have an ability to specify multiple -focus and -skip flags. Here's why.

Currently, I have something like this in one of the CI files:

-skip=[Slow]|[Serial]|[Disruptive]|[Flaky]|[Feature:.+]|PersistentVolumes|[HPA]|should.support.building.a.client.with.a.CSR|should.propagate.mounts.to.the.host|for.NodePort.service|type.clusterIP|unready.pods|ExternalName.services|Guestbook.application|in-cluster.config|Pods.should.support.pod.readiness.gates|[sig-storage].In-tree.Volumes.[Driver:.local]|[sig-storage].CSI.Volumes.CSI.Topology.test.using.GCE.PD.driver|[sig-storage]\sCSI\sVolumes\s[Driver:\scsi-hostpath]\s[Testpattern:\sDynamic\sPV\s(block\svolmode)]\svolumes\sshould\sstore\sdata|[sig-storage]\sCSI\sVolumes\s[Driver:\scsi-hostpath]\s[Testpattern:\sDynamic\sPV\s(block\svolmode)]\sprovisioning\sshould\sprovision\sstorage\swith\spvc\sdata\ssource|[sig-network]\sServices\sshould\sbe\sable\sto\spreserve\sUDP\straffic\swhen\sserver\spod\scycles\sfor\sa\sNodePort\sservice|Events.should.be.sent.by.kubelets.and.the.scheduler.about.pods.scheduling.and.running|[sig-apps].DisruptionController.should.block.an.eviction.until.the.PDB.is.updated.to.allow.it|[sig-instrumentation].MetricsGrabber

and yes, it's a mess. This, though, can be made much less messy by having something like this instead:

       -skip=\[Slow\]
       -skip=\[Serial\]
       -skip=\[Disruptive\]
       -skip=\[Flaky\]
       -skip=\[Feature:.+\]
       -skip=PersistentVolumes
       -skip=\[HPA\]
       -skip=should.support.building.a.client.with.a.CSR
       -skip=should.propagate.mounts.to.the.host
       -skip=for.NodePort.service
... and so on ...

Implementation-wise it seems quite doable, I think I'll have a PR soon. Hope that would be usable.

Similar idea was already proposed in #550 (comment)