build: Update test/verification section of PR template

[tharapalanivel]

Description of the change

Update test/verification section of PR template to enforce tests (in the case that unit tests can't be added, an explanation should be added)

Related issues or PRs

(Optional) List any documentation or testing added

(Optional) How to verify the contribution

Checklist for passing CI/CD:

• All commits are signed showing "Signed-off-by: Name <email@domain.com>" with git commit -signoff or equivalent
• PR title and commit messages adhere to Conventional Commits
• Contribution is formatted with tox -e fix
• Contribution passes linting with tox -e lint
• Contribution passes spellcheck with tox -e spellcheck
• Contribution passes all unit tests with tox -e unit

Note: CI/CD performs unit tests on multiple versions of Python from a fresh install. There may be differences with your local environment and the test environment.