add subPath e2e test

[boddumanohar]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
add subPath e2e test

Which issue(s) this PR fixes:
Fixes #162

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

none

[k8s-ci-robot]

Hi @boddumanohar. Thanks for your PR.

I'm waiting for a kubernetes-csi member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[andyzhangx]

/ok-to-test

[coveralls]

Pull Request Test Coverage Report for Build 726101432

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 80.47%

---

Totals
Change from base Build 724496378:	0.0%
Covered Lines:	548
Relevant Lines:	681

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💛 - Coveralls

[msau42]

@andyzhangx instead of writing custom tests, can we reuse the kubernetes e2e testsuites? You get 50+ test cases for free. https://github.com/kubernetes/kubernetes/tree/fa558043c02cae10ffbe001b46a7166231d07f60/test/e2e/storage/external

[andyzhangx]

@andyzhangx instead of writing custom tests, can we reuse the kubernetes e2e testsuites? You get 50+ test cases for free. https://github.com/kubernetes/kubernetes/tree/fa558043c02cae10ffbe001b46a7166231d07f60/test/e2e/storage/external

thanks for reminder, since this case is already done, let's merge it first.
I have a plan to reuse the kubernetes e2e testsuites in this project, will work on that later.

[andyzhangx]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: andyzhangx, boddumanohar

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