[Test] Remove code duplication, use getCredentials method

tests/data/api-data.js

tests/end-to-end/api/00-miscellaneous.js

tests/end-to-end/api/01-users.js

tests/end-to-end/api/02-channels.js

tests/end-to-end/api/03-groups.js

tests/end-to-end/api/04-direct-message.js

tests/end-to-end/api/05-chat.js