make pmbootstrap test cases work independently of pmaports changes
The Qemu test case always does a full installation of XFCE4 and Plasma Mobile. Right now it does not run through because the current pmaports version of Plasma Mobile is not built yet in the binary package repository (pmaports#28 (closed)).
It would be good if the pmbootstrap test cases worked independently of that, and we can achieve this by tagging the pmaports repository whenever we raise the minimum pmaports version that pmbootstrap needs (as defined in
pmb/config/__init__.py), and then downloading that tag instead of master when running the pmbootstrap tests.