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.
