tests: Add a way for the user to skip selenium tests

tests: Add a way for the user to skip selenium tests

It's a bit more convoluted to setup the test environment to work with
selenium (eg. needs a browser installed and Xorg running, ...). It's
possible someone would want to skip those.

v2: Use 'PW' prefix rather than 'PATCHWORK', per existing test
  parameters, and pass said variable through tox

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>