test: fix flaky qsync_with_anon.test.lua again

One of the test cases had 2 problems.

- The same as in the previous commit - it started a sync
  transaction on master, switched to replica assuming it sees
  everything up to this sync transaction, but it still can see
  data from the previous test case;

- The test case tried to write a sync transaction on master, got
  timeout, switched to replica to ensure the data is removed here
  too, but since dirty reads are possible, it could happen the
  data was delivered to replica and ROLLBACK wasn't not yet. On
  the replica the rolled back data still could be visible.

The first issue is solved by flushing master's state to replica
via making a successful sync transaction.

The second issue is fixed by splitting it into more steps, not
depending on timeouts (1000 is considered infinity).

Closes #5196