[odoo] tests of webkit reports

[Georges Racinet]

Some addons (account) have tests that involve actual rendering of webkit based reports.
These require a working wkhtmltopdf (preferably >= 0.12) and the setting the DISPLAY environment variable to point on an available X server (the typical buildslave being headleass, this will be a xvfb instance in most setups).

We'll need a new capability to indicate the availability of wkhtmltopdf, with a 'display' option to indicate the X server, if different from the default :0

Example:

    wkhtmpltopdf 0.12 display=:1

Note for Debian: wkhtmltopdf 0.12 landed in wheezy-backports a week ago.

[Georges Racinet (gracinet)]

Done, the version number is mandatory to have the DISPLAY environment variable correctly set.
Examples:

   wkhtmltopdf 0.12
   wkhtmltopdf 0.12 display=:1

[Georges Racinet (gracinet)]

And also, the wkhtmltopdf capability must be explicitely declared in build-requires to trigger the environment variable setting.