tests: support more file systems in the cp fiemap tests
* tests/cp/sparse-fiemap: Check for fiemap support against a file
rather than a directory to enable tests on BTRFS for example.
Explicity disable the test on ext3 or file systems where we
can't determine the type.
* tests/cp/fiemap-perf: Likewise. Also disable the test on older
BTRFS (like in Fedora 14), where extents are returned for holes.
* tests/init.cfg: Comment that BTRFS only supports fiemap
for regular files.