tests: determine if SEEK_HOLE is enabled

tests: determine if SEEK_HOLE is enabled

Upcomming gnulib changes may disable SEEK_HOLE
even if the system supports it, so dynamically
check if we've SEEK_HOLE enabled.

* init.cfg (seek_data_capable_): SEEK_DATA may be disabled in the build
if the system support is deemed insufficient, so also use `cp --debug`
to determine if it's enabled.
* tests/cp/sparse-2.sh: Adjust to a more general diagnostic.
* tests/cp/sparse-extents-2.sh: Likewise.
* tests/cp/sparse-extents.sh: Likewise.
* tests/cp/sparse-perf.sh: Likewise.