tests: fix async allocation race on BTRFS

tests: fix async allocation race on BTRFS

* tests/dd/sparse.sh: Sync files before checking allocations,
which may be done asynchronously on NFS and BTRFS at least.
Also mark this test as very expensive on remote file systems.
* tests/du/2g.sh: Likewise, also use fallocate if available
to efficiently allocate the large file, otherwise skip
on remote file systems.
* tests/tail-2/inotify-rotate-resources.sh: Use the more
standard is_local_dir_() to check remoteness.
* tests/cp/fiemap-empty.sh: Comment on the sync issue
for this currerntly unused test.
Fixes http://bugs.gnu.org/20570