test: thinpool auto-extend test

[dsharma-dc]

This change adds a test that verifies a thinpool auto-extends in size as configured in the lvm.conf file.

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Why is this PR required? What issue does it fix?: This adds a test for verifying thinpool auto-extend.

What this PR does?: Adds a new test which fills up the LVM logical volume to a certain capacity so that auto-extend of underlying thinpool kicks in.

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>
• [x] 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: