Create tests for Milestone methods

[allardbrain]

These endpoints in the ClubhouseClient class in api.py need to have tests created:

• create_milestone()
• delete_milestone()
• get_milestone()
• list_milestone_epics()
• list_milestones()
• update_milestone()

When tests should fail

• The request URL did not match the URL for the endpoint
• The HTTP method in the request did not match what is required by the endpoint
• Required argument(s) was(were) not provided
• Argument(s) was(were) not of the right type
• An integer supplied as an argument is smaller than 1

When tests should succeed

• The request URL matched the URL for the endpoint
• The HTTP method in the request matched what is required by the endpoint
• The required argument(s) was(were) provided
• Argument(s) was/(were) of the correct type
• An integer supplied as an argument is equal to or greater than 1