tests: ensure we test SEEK_DATA where used

tests: ensure we test SEEK_DATA where used

fiemap is no longer the default copy implementation,
so check for SEEK_DATA support instead as that's preferred.
This will ensure better test coverage on systems without fiemap.

* init.cfg: Replace fiemap_capable_ with seek_data_capable_.
This is best supported with python 3 so prefer that.
* tests/seek-data-capable: A new test script checking for
SEEK_DATA support on the passed file name,
called from seek_data_capable_.
* tests/fiemap-capable: Remove no longer used probing script.
* tests/cp/fiemap-perf.sh: Renamed to tests/cp/sparse-perf.sh
* tests/cp/fiemap-2.sh: Renamed to tests/cp/sparse-2.sh
* tests/cp/fiemap-extents.sh: Renamed to tests/cp/sparse-extents.sh
* tests/cp/sparse-fiemap.sh: Renamed to tests/cp/sparse-extents-2.sh
* tests/cp/fiemap-FMR.sh: Renamed to tests/cp/copy-FMR.sh
* tests/local.mk: Reference the renamed tests.