test: pytestified test_cc_growpart.py

[MoeSalah1999]

Proposed Commit Message

Converted test_cc_growpart.py from unittest to pytest:
-Removed all unittest and TestCase imports and inheritances.
-Used pytest fixtures.
-Converted all self.assert to plain assert.
-Made a couple of minor changes to avoid unusual behavior.

Fixes GH-6427 

Additional Context

Test Steps

Merge type

• [x] Squash merge using "Proposed Commit Message"
• [ ] Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)