The first PdfHandler Selenium tests

See also commits in PdfHandler[1], jenkins-job-builder-config[2] and zuul-config[3].

1: https://gerrit.wikimedia.org/r/#/c/143175/
2: https://gerrit.wikimedia.org/r/#/c/152246/
3: https://gerrit.wikimedia.org/r/#/c/152788/

Change-Id: I974c7631bab3f81c845898a0fbc3026a8ef18e5b