Proper directory layout for tests
There are three different testing frameworks in Drupal 8.

1. Simpletest: Functional, Upgrade and Unit Tests

These should be located underneath src\Tests per the documentation linked in the Question comments. This corresponds to the namespace Drupal\my_module\Tests.

This is documented best at https://www.drupal.org/node/2166895.

2. PHPUnit: Unit Tests

Theoretically these could be anywhere with a phpunit.xml.dist file, but if you need to support running through Drupal's test runner (run-tests.sh) or drupal.org infrastructure then these must be placed in tests\src\Unit and the namespace corresponds to Drupal\Tests\my_module\Unit

3. Behat: Behavioral Tests

I think there is a movement to store behavioral tests in a directory parallel to tests\src\unit, but no official convention as Drupal core does not utilize Behat yet.