Unnecessary `testInboxId` validation in testing

[i7an]

testInboxId is not always needed.

client.testing.projects.create('test project') // => MailtrapError: testInboxId is missing, testing API will not work.
client.testing.inboxes.create(123, 'test inbox') // => MailtrapError: testInboxId is missing, testing API will not work.

[narekhovhannisyan]

Hi @i7an, this should be covered by #70

[i7an]

@narekhovhannisyan thanks!