Add end to end checkout tests

[pmclain]

GraphQL response contains errors: Guest email for cart is missing. Please enter

Tests checkout flow for customer and guest.

Description (*)

Tests checkout flow for customer and guest.

Guest checkout currently fails with the error: GraphQL response contains errors: Guest email for cart is missing. Please enter Fixed by #564

Fixed Issues (if relevant)

1. [Test coverage] End to tests for customer checkout workflow #604

Manual testing scenarios (*)

1. cd dev/tests/api-functional && ../../../vendor/bin/phpunit testsuite/Magento/GraphQl/Quote/Customer/EndToEndCheckoutTest.php

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

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