tests: Fix test_evpn_mh.py to correctly call bridge program

tests: Fix test_evpn_mh.py to correctly call bridge program

Getting this error:

2024-01-17 19:05:20,688 WARNING: torm11: Router(torm11): proc failed: rc 255 pid 2436134
	args: /usr/bin/nsenter --mount=/proc/2435168/ns/mnt --net=/proc/2435168/ns/net --uts=/proc/2435168/ns/uts -F /bin/bash -c /sbin/bridge vlan add vid 1000 dev bridge
	stdout: RTNETLINK answers: Operation not supported
	stderr: *empty*

As I understand it the correct thing to do here is pass in:
bridge vlan add vid 1000 dev bridge self

Signed-off-by: Donald Sharp <sharpd@nvidia.com>