Re-enable the Integration Tests in xfail mode

[cclauss]

Instead of getting rid of our Integration Tests as discussed in #2725, let's re-enable them and set the ones that are not working in pytest.mark.xfail mode so that they can be fixed over time.

This approach allows us to keep the eight tests that already work and add new tests to cover the new Python 3 code.

This PR also adds a new Travis CI job to automatically run these user interface tests on all pull requests. The problem is that building the Docker images from scratch takes 8+ minutes on Travis CI so the total job takes 12 minutes! Perhaps this PR should be landed after #3615 so we can reuse that Docker Hub image.

Technical

Testing

The testing steps are discribed at
https://github.com/internetarchive/openlibrary/pull/3674/files#diff-6b1c0422e0532f9de8b00ee73de3d6acR21

Screenshot

Stakeholders

pushd tests/integration
pytest .

================================ test session starts ======================================
platform darwin -- Python 3.8.5, pytest-6.0.1, py-1.9.0, pluggy-0.13.1
rootdir: /Users/cclauss/Python/itinerant_futurizer/openlibrary
collected 44 items

test_affiliate_links.py ..                                                                                                                                                                                                                                                        [  4%]
test_auth.py .xxxxxxxxxxxxxx.xxxxxxx.                                                                                                                                                                                                                                             [ 59%]
test_landing.py xxxxx                                                                                                                                                                                                                                                             [ 70%]
test_loans.py xxx                                                                                                                                                                                                                                                                 [ 77%]
test_microdata.py x.                                                                                                                                                                                                                                                              [ 81%]
test_search.py .xxx                                                                                                                                                                                                                                                               [ 90%]
test_sharing.py xx.x                                                                                                                                                                                                                                                              [100%]

=========================== 8 passed, 36 xfailed in 169.30s (0:02:49) ===========================

[cclauss]

Closed in favor of #3682