unittests.cfg: groups should be space separated

As specified in the comment blocks at the tops of the unittests.cfg
files, multiple groups assigned to 'groups' should be space separated.
Currently any nonword character works for the deliminator, but the
implementation may change. Stick to the specs.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20211112133739.103327-2-drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>