Tests for active clients endpoint

[nicole-mcg]

Tests should be created for the ActiveClientsEndpoint class in file_server/web/endpoints/active_clients.py

You can create a test server for endpoints with create_test_server in file_server/util/test_util.py. An example test can be found at file_server/web/endpoints/__test__/test_directory_contents.py

The following tests should be created:

• Verify returns correct response with a single active client
• Verify response is correct length, and has correct properties with multiple active clients

Test naming conventions: https://pytest.readthedocs.io/en/reorganize-docs/new-docs/user/naming_conventions.html