test: index_with_paging_test: increase semaphore's queue size
test: index_with_paging_test: increase semaphore's queue size

To allow the flood of reads generated by this test to be queued up
during up-front admission without failing the test.

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