Add more test coverage for shell formatter
Add more test coverage for shell formatter

.add_argument_group() was not covered by tests, nor was  parameter "--prefix"

Change-Id: If757613793201defa3d9d7006d3a4a0a2a9a43c2
Signed-off-by: Vincent Legoll <5d42a0e44e2b47374e182c98b792e43fa0a82956@idgrilles.fr>

dtroyer/cliff,openstack/cliff,openstack/cliff,dtroyer/cliff