tests: create 128 partitions also in the scsi-backed test

tests: create 128 partitions also in the scsi-backed test

* tests/t9040-many-partitions.sh: Tighten up: use less memory:
1 sector per partition rather than 256.
Exec parted just once, not once per partition.
With that, we can increase the number of partitions to create
from 60 (which used to take 2.5 minutes on an F12-era kernel)
to the standard-GPT-header-imposed maximum of 128.
Suggested by Phillip Susi.