Add smoke tests

[tellyworth]

This adds some basic smoke tests (mis)using phpunit.

The tests will fetch http://localhost:8888 and http://localhost:8888/download/ and run a few basic sanity tests on the HTML output.

Sample output:

% yarn test
yarn run v1.22.18
$ ./vendor/phpunit/phpunit/phpunit tests/phpunit
PHPUnit 7.5.20 by Sebastian Bergmann and contributors.

........SS                                                        10 / 10 (100%)

Time: 6.05 seconds, Memory: 4.00 MB

OK, but incomplete, skipped, or risky tests!
Tests: 10, Assertions: 12, Skipped: 2.
✨  Done in 6.17s.

How to test the changes in this Pull Request:

1. yarn test

[ryelle]

Is this meant to be a framework for other tests? It works for me, but I'm not totally sure why we need it.

[tellyworth]

Intended as a starting point, and potentially a framework for more tests if it's useful.

It probably isn't useful though if there's not a good way to incorporate it into the sync/deploy process. Probably belongs in next iteration / pending discussion, to figure out if it makes sense.