test: add 'eventually' block to index paging test
test: add 'eventually' block to index paging test

Without 'eventually', the test is flaky because the index can still
be not up to date while checking its conditions.

Fixes #4670

scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla