tests/discard: Ensure a significant number of blocks are freed up on the host.

It's reasonable to expect megabytes will be freed up and given back to
the host.  Previously we just tested that at least 1 block was
released.  This test checks that at least 1000 blocks are released
(since stat(2) returns blocks of 512 bytes: 512 bytes * 1000 = approx 0.5 MB).
tests: fstrim: Remount the disk.

This makes fstrim work.  It's not clear why exactly.