test: fix flaky qsync_basic.test.lua

In one of the test cases 2 fibers were started making a
transaction. In the first fiber the transaction was rolled back,
and the second fiber was expected to do the same.

It did rollback too, but not always immediately after the first
one. Because the first fiber needed not just do rollback right
away, but write a ROLLBACK entry into WAL before applying the
rollback to all next transactions. This led to a yield, during
which it was possible to observe the second fiber not dead yet.

The patch makes the test explicitly wait for the fibers death.

Closes #5162