Add hack/verify-coverage.sh

[mattcary]

What type of PR is this?
/kind feature

What this PR does / why we need it:
Add a coverage script to hack/. In addition to making it easier to manually verify code coverage, this will enable automated tooling to produce coverage reports.

Running it looks like

$ ./hack/verify-coverage.sh 
Verifying coverage
ok  	sigs.k8s.io/gcp-compute-persistent-disk-csi-driver/pkg/common	0.095s	coverage: 66.7% of statements
ok  	sigs.k8s.io/gcp-compute-persistent-disk-csi-driver/pkg/gce-cloud-provider/compute	0.134s	coverage: 3.4% of statements
ok  	sigs.k8s.io/gcp-compute-persistent-disk-csi-driver/pkg/gce-pd-csi-driver	0.212s	coverage: 58.6% of statements

(gce-cloud-provider looks pretty sad... it's possible we'll want to exclude it from coverage if it's not unit-testable. This can be refined after we look more closely at the coverage data)

None

[mattcary]

/hold

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mattcary

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[mattcary]

/unhold
/assign @saikat-royc
/unassign @saad-ali

[saikat-royc]

/lgtm