Make PUBLISHER_ENDPOINT constant local to the test
Make PUBLISHER_ENDPOINT constant local to the test

For some reason the tests started failing claiming they couldn't find a constant
that had been included into the current scope through include GdsApi::TestHelpers::Publisher
Declaring the constant locally makes the tests pass.

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