enable dnf_warning in mock config

Fixes: https://pagure.io/koji/issue/1594

tests/test_lib/data/mock/simple.out

@@ -3,6 +3,7 @@
 config_opts['basedir'] = '/var/lib/mock'
 config_opts['chroot_setup_cmd'] = 'groupinstall build'
 config_opts['chroothome'] = '/builddir'
+config_opts['dnf_warning'] = True
 config_opts['root'] = 'ROOTNAME'
 config_opts['rpmbuild_networking'] = False
 config_opts['rpmbuild_timeout'] = 86400