test: refactor integration test directory structure and improve test command usage

[ykim-1]

📝 Description

This PR aims to refactor test directory structure to improve readability and usage of make commands

Updated test make commands are given below:

make TEST_SUITE=linode_client testint         // Runs tests from `test/integration/linode_client`

make TEST_SUITE=account testint         // Runs tests from `test/integration/models/account`

make TEST_CASE=test_get_domain_record testint        // Runs single test case

✔️ How to Test

What are the steps to reproduce the issue or verify the changes?

How do I run the relevant unit/integration tests?

📷 Preview

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[jriddle-linode]

LGTM, works locally.

[yec-akamai]

Nice refactoring!