test creating sized snapshots for filesystem and block volumes

[dsharma-dc]

Pull Request template

Please, go through these steps before you submit a PR.

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 #
• 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.