testpackages: update for rpmfluff change, use pandoc-common
I, uh, forgot this script existed and did these changes by hand
when I changed the openQA tests to use pandoc-common. Let's
retcon the script! rpmfluff changed where `YumRepoBuild` lives,
and we need to build a fake pandoc-common now.
Signed-off-by: Adam Williamson <email@example.com>