Description of problem:
The old yum supports the following syntax (formerly part of the yum-security plugin, which was merged into the yum core at some point) to update all packages from a given update group:
yum --enablerepo=updates-testing --advisory=FEDORA-2015-15543 update
Unfortunately, it looks like dnf does not support that. (At least it is not documented.) This is an essential feature for testing large update groups with many packages and even more subpackages. (The command lines suggested by Bodhi will usually NOT work because Bodhi does not enumerate the subpackages. There is a longstanding RFE to make it use --advisory instead, but dnf needs to support it first.)