test: fix flaky gh_7515_sync_node_sees_leader_hang

The test checks, whether the instance notices leader hang during
sync. For that it stops server2 and generates some data on the
master (server1). The problem is, when connection to server3
flakes, server1 resigns and becomes read only due to fencing
enabled.

Let's disable fencing for this test, as we don't want automatic
leader resigning here.

Closes tarantool/tarantool-qa#325

NO_CHANGELOG=test
NO_DOC=test