Fix katello-agent pre-upgrade warning

tests/foreman/destructive/test_katello_agent.py

@@ -153,8 +153,7 @@ def test_positive_upgrade_warning(sat_with_katello_agent):
 
     """
     sat = sat_with_katello_agent
-    ver = sat.version.split('.')
-    target_ver = f'{ver[0]}.{int(ver[1]) + 1}'
+    target_ver = '6.15'
     warning = (
         'The katello-agent feature is enabled on this system. As of Satellite 6.15, katello-agent '
         'is removed and will no longer function. Before proceeding with the upgrade, you should '
@@ -167,14 +166,11 @@ def test_positive_upgrade_warning(sat_with_katello_agent):
         'which will automatically uninstall katello-agent.'
     )
 
-    upstream_rpms = sat.get_repo_files_by_url(constants.FOREMAN_NIGHTLY_URL)
-    fm_rpm = [rpm for rpm in upstream_rpms if 'foreman_maintain' in rpm]
-    assert fm_rpm, 'No upstream foreman-maintain package found'
-
-    for rpm in fm_rpm:
-        res = sat.execute(f'yum -y install {constants.FOREMAN_NIGHTLY_URL}{rpm}')
-        assert res.status == 0, f'{rpm} installation failed'
+    settings.repos.satellite_version_undr = '6_15'
+    settings.repos.snap_version_undr = '6_15_0-13_0'
+    sat.create_custom_repos(next_maintain=settings.repos.NEW_MAINTENANCE_REPO)
 
+    sat.cli.Upgrade.list_versions()  # f-m self-update
     res = sat.cli.Upgrade.list_versions()
     assert res.status == 0, 'Upgrade list-versions command failed'
     assert target_ver in res.stdout, 'Target version or Scenario not found'