tests: avoid nasty FP-causing race condition

tests: avoid nasty FP-causing race condition

* tests/t9040-many-partitions.sh: Remove all partitions before
exiting -- otherwise, the rmmod-prompted removal of those many
partitions would continue long after rmmod completion and the
release of the scsi_debug lock, and thus partition-device removals
provoked by this test would cause failure in the subsequent test,
e.g., when the devices for partitions 14,15,16 have just been
successfully created (verified that they exist), yet the next
step -- to create a file system on one of them -- fails or is
erroneously skipped because the device has been removed asynchronously.