tests: Make wpas_mesh_max_peering more robust

tests: Make wpas_mesh_max_peering more robust

The previous version was enabling all three stations at the same time
and left dev[1] and dev[2] competing on getting connected with dev[0]
that allowed only one pairing. This was not exactly robust and the pass
criteria depended on an extra event from either dev[1] or dev[2]. Fix
that by first connecting dev[0] and dev[1] and only after that, start
dev[2]. This allows proper validation of both the peering limit on
dev[0] and no extra event on dev[2].

Signed-off-by: Jouni Malinen <j@w1.fi>