test: Update smoke test coverage and improve nightly test workflow

[ykim-akamai]

📝 Description

This PR updates smoke test coverage and improve nightly smoke test workflow:

• Test coverage updated
• Added option to test specific branch with sha input
• Added slack notifications at end of nightly smoke test workflow

✔️ How to Test

make smoketest

Forked run - https://github.com/ykim-1/linode-cli/actions/runs/10585711307

📷 Preview

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