Add a basic test of apertis-pkg-merge-updates functionality

This will try to merge an "update" of dash on a completely made-up
package history. There are two fixtures, sharing common parts. They
both:

* Start with an ancient dash version from Lenny, 0.5.4-12.
* This version is imported as 0.5.4-12co0 into an imaginary
  Apertis version.
* A "new" version of dash, 0.5.10.2-1 is imported from a 2018
  cut of Buster.
* This new version is merged into Apertis as 0.5.10.2-1co0

From this point, the "modified" fixture differs:
* An imaginary Apertis developer changes the compat level in
  the imported version from 10 to 11, thus introducing a delta
  which however is not going to conflict with any further version
  of dash in Buster.

apertis-pkg-merge-updates is expected to merge the update into the
"unmodified" repository as 0.5.10.2-5co0 and finalise the changelog.

For the "modified" repository, it should import the update as
0.5.10.2-5co1, add "PLEASE SUMMARIZE remaining Apertis changes" note
to the top entry and leave it UNRELEASED.

Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk>