How do you unit test plugins?
Have you been through Craft's testing documentation? https://craftcms.com/docs/3.x/testing/
There's a pretty good walkthrough of the necessary steps there to test plugins: https://craftcms.com/docs/3.x/testing/testing-craft/setup.html
Or if you like more real-world examples, Commerce's tests are a good place to start: https://github.com/craftcms/commerce/blob/develop/codeception.yml
From then, you'd trigger all unit tests from the root of your project like:
./vendor/bin/codecept run unit

Or every test in a single file with:
./vendor/bin/codecept run tests/unit/helpers/LocaleHelperTest.php

Or a specific test within a single file with:
./vendor/bin/codecept run tests/unit/helpers/LocaleHelperTest.php:testEmailGetOrderLanguage