Changes

repo-install-test: fix test_conflict for debian 13

Two changes are necessary to make this test work with debian 13:

* Installing libosmocore from osmocom-latest, then switching to
  osmocom-nightly and attempting to install another package is not
  enough anymore to trigger a conflict. apt is now able to resolve this
  by uninstalling the osmocom-latest package and upgrading libosmocore
  to the nightly version. Force the conflict by explicitly marking
  osmocom-latest (osmocom-$FEED) as installed and for hold.

* The apt conflict message has been reworked, so the string to look for
  needs to be adjusted.

Change-Id: Ibdcd583e48b97ced11ad4939974dccea3e139480