References to group_test_content in Group 2.x

Problem/Motivation

Trying to upgrade from Group 1.6x to 2.x, and I'm finding some breaking references to `group_test_content` that I suspect shouldn't be included.

Steps to reproduce

- running the following:

```bash
composer require 'drupal/groupmenu:^3.0@alpha' 'drupal/group:^2.3
```

- I get the following failure message from drush watchdog

- `Fatal error: Cannot declare class Drupal\group_test_content\Entity\StringContent, because the name is already in use in /code/web/modules/contrib/group/src/Entity/StringContent.php on line 24`
- `Fatal error: Cannot declare class Drupal\group_test_content\Entity\StringConfig, because the name is already in use in /code/web/modules/contrib/group/src/Entity/StringConfig.php on line 21`

Proposed resolution

- Doing a grep on that string `group_test_content`, I'm finding three files seemingly still reference.

```bash
web/modules/contrib/group/src/Entity/StringConfig.php:3:namespace Drupal\group_test_content\Entity;
web/modules/contrib/group/src/Entity/StringContent.php:3:namespace Drupal\group_test_content\Entity;
web/modules/contrib/group/src/Entity/IntegerContent.php:3:namespace Drupal\group_test_content\Entity;
```

- There is also like some `group/tests/modules/group_test_content`,
- along with `group/modules/group_test_content`
- and also `group/b/tests/modules/group_test_content`

It seems like there is a lot of drift for testing suites in the group module!