tests: fix ldp_vpls_topo1 to work as expected

tests: fix ldp_vpls_topo1 to work as expected

In the last step of this test, r1's link to r2 is shut down but
both routers stay connected through a multi-hop LDP session. That
happens because r1 and r2 have a targeted adjacency created by
the pseudowire.  The test then checks whether the pseudowire is
still up, using an alternate path for nexthop resolution.

Everything's fine except for the fact that LDP GTSM (aka
ttl-security) is enabled by default. This means that messages sent
over a multi-hop session are not delivered. In the case of this
test, it can prevent PW-Status notifications from being delivered,
which in turn can prevent the pseudowire from coming back up.

Fix the test by disabling GTSM so that LDP multi-hop sessions can
work normally. This is in accordance with RFC6720 which mentions
that GTSM should be disabled (statically or dynamically) for
multi-hop sessions.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>