Fix and re-enable the E2E tests

[nylen]

The E2E (end-to-end) tests to check the overall behavior of CC using an automated browser are not working correctly. Example: https://travis-ci.com/ClassicPress-research/classic-commerce/jobs/278382607

In #174 they were moved to "allowed to fail" on Travis so that we can work with the build passing, but these need to be fixed and re-enabled.

Woo has made a lot of changes to this system, we'll probably want to incorporate some/most of them.