tests: remove unused class attribute in VppTestCase
tests: remove unused class attribute in VppTestCase

Identified in post-merge review.

Type: test

Change-Id: I46e19285479437561a43975ba9b5cb68f478736c
Signed-off-by: Paul Vinciguerra <b92f79aabe4c9c18085c7347110a52af0898a0ef@vinciconsulting.com>

vpp-dev/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,vpp-dev/vpp,vpp-dev/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,vpp-dev/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,vpp-dev/vpp,vpp-dev/vpp