testing/varnish-modules: new package

add a varnish-modules package to provide essential modules not included in the Varnish packages.

Two notes:

• the patch will go away mid-september when we get a new release (varnish-modules follow the same release cycle as varnish, mid-september and mid-march). The issue is tracked upstream here: https://github.com/varnish/varnish-modules/issues/187
• modules need to be build against the same Varnish version that then'll run on, so I used a dirty trick to ensure $pkgver is the same, but there may be a cleaner way