test creating sized snapshots for filesystem and block volumes

[dsharma-dc]

Pull Request template

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Why is this PR required? What issue does it fix?: Exercises the snapSize behaviour of volume snapshot class.

What this PR does?: Tests creating sized snapshots for filesystem and raw block volumes.

Does this PR require any upgrade changes?: No

If the changes in this PR are manually verified, list down the scenarios covered::

Any additional information for your reviewer? : Mention if this PR is part of any design or a continuation of previous PRs

Checklist:

• [ ] Fixes #
• [x] PR Title follows the convention of <type>(<scope>): <subject>
• [ ] Has the change log section been updated?
• [ ] Commit has unit tests
• [ ] Commit has integration tests
• [ ] (Optional) Are upgrade changes included in this PR? If not, mention the issue/PR to track:
• [ ] (Optional) If documentation changes are required, which issue on https://github.com/openebs/openebs-docs is used to track them:

[dsharma-dc]

This PR is based on top of branch used for PR 308 and hence we see that commit here as well. That commit doesn't need review against this PR.