IMMINENCE_API_HOST => IMMINENCE_API_ENDPOINT
IMMINENCE_API_HOST => IMMINENCE_API_ENDPOINT

This means that we use the protocol that Plek chooses, rather than
inconsistently stubbing http, https or both.

It also makes it consistent with the other test helpers.

theodi/gds-api-adapters,alphagov/gds-api-adapters,theodi/gds-api-adapters,bitzesty/gds-api-adapters,bitzesty/gds-api-adapters