testcase for  sign_client.client.py

**Description**
new file test_client.py added for sign_client.client.py

**Describe the solution you'd like**
An added a new test case in the test_client.py file.

1. test_base_uri()
2. test_get_users()
3. test_get_groups()
4. test_update_sign_users()
5. test_update_existing_users()

