tests: fix a false positive fiemap test on some file systems

tests: fix a false positive fiemap test on some file systems

* tests/filefrag-extent-compare: Don't check the length of the
last extent, as this was seen to vary on XFS, where it leaves
trailing blocks allocated for performance reasons.
* tests/cp/fiemap-empty: Though not seen as an issue in practise,
try to avoid possible issues with the allocator in file systems,
by requesting to allocate a power of 2.