Ignore mocks in code coverage report

[AndrewSirenko]

Is this a bug fix or adding new feature?
bug fix

What is this PR about? / Why do we need it?
Currently our code coverage tool's per-package code-coverage is inaccurate due to our mock files being tracked by go test -coverprofile.

By appending _test to each mock file, go test will know that these files should not be counted towards the code-coverage score.

What testing is done?
make mockgen succeeds

NOTE: make verify fails when verifying govet with the following error:

Verifying govet
# github.com/kubernetes-sigs/aws-ebs-csi-driver/pkg/driver
vet: pkg/driver/mock_k8s_client_test.go:66:6: MockKubernetesClient redeclared in this block
# github.com/kubernetes-sigs/aws-ebs-csi-driver/pkg/cloud
vet: pkg/cloud/mock_cloud_test.go:16:6: MockCloud redeclared in this block

But these files should be handled by make mockgen

[k8s-ci-robot]

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