Why is my unit test not detected?
Unit tests must have the following namespace: Drupal\Tests\dev_module\Unit.

For complicated reasons, they also don't show up in the right group but they are all in the PHPUnit group.

To run your unit tests with phpunit, just pass the module folder, that also makes it 10x faster for finding the tests:

./vendor/bin/phpunit -c core modules/dev_module