test: fix flaky downstream lag test

It could fail in ASAN build. Can't tell why just there.

The main reason was that in a topology server1 + server2->server3
one of the cases
- did a txn on server1,
- then enabled server2->server3 replication,
- then waited for server2->server3 sync,
- and instantly assumed the txn reached server3.

Surely it not always did. At the server2->server3 sync the txn
might not had reached server2 itself yet.

The fix is as simple as explicitly ensure the txn is on server2
before waiting server2->server3 sync.

Another potential for flakiness was that the default timeout in
luatest.helpers.retrying is super low, just 5 seconds. The patch
manually bumps it to 60 seconds to be sure any future failures
wouldn't be related to too small timeout.

Closes #10031

NO_DOC=test
NO_CHANGELOG=test