ApiMockServer adds missing mocks to mock state

[skovmand]

When using the ApiMockServer during testing a problem can occur, when a call is made which is not properly mocked.

For example if during a test we call: ApiClient.get_households() without setting up a mock response first.

The return value will correctly be set to:
{:error, "No mock set for call", get_households, %{query: %{}}}

However the state in the ApiClientMockServer will be updated with the new state for mocks:
{:get_households, %{query: %{}} => nil

This means that all following calls to assert that no mocks remain will fail:
assert ApiMock.remaining_calls() == %{}

The correct behavior is to return the no mock set error, and leave the state unchanged.

[skovmand]

TODO:

• First priority is to make tests that prove that the bug exists.
• Fix the problem (which will be a refactor of the involved GenServer function).