Revert "tests: fstrim: Remount the disk."

This reverts commit accf1b66aa835714690a2979e990c49243875dab.
tests: fstrim: Sync before running fstrim.

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

Thanks: Paolo Bonzini.
tests/discard: sync before close isn't necessary.

However we should be calling shutdown, since we're writing to the disk
and need to catch qemu errors.