Add kube client oidc authentication support for testing

[zhangtbj]

Changes

Currently, the test client in Tekton test doesn't support Kubernetes cluster with oidc authentication.
I tried to run e2e test for my Tekton build, and I found the test can only use the kubeconfig which in ~/.kube/config, and my kube cluster use oidc for auth but the current Tekton client doesn't support that.

I take a look the knative serving and eventing, their clients have the config for oidc:
https://github.com/knative/serving/blob/78995f561c50acb87c9f5d386e07dbb3c597aeeb/test/clients.go#L25

But tekton test client doesn't have that:
https://github.com/tektoncd/pipeline/blob/master/test/clients.go

Without this oidc package,all tests will fail like:

--- FAIL: TestEventListenerCreate (0.00s)
    eventlistener_test.go:48: Failed to create kubernetes clientset from config file at /home/travis/.kube/config: No Auth Provider found for name "oidc"

So I follow what knative repo did in this PR:
1, Use dep to add oidc package:
dep ensure -add k8s.io/client-go/plugin/pkg/client/auth/oidc

2, Add import in test/init_test.go

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[tekton-robot]

Hi @zhangtbj. Thanks for your PR.

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[zhangtbj]

Hi @vdemeester ,

Pls review by using this one. Thanks!

[vdemeester]

/ok-to-test

[vdemeester]

/lgtm
Thanks @zhangtbj

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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