tests/bench-slope: use same benchmarking for XTS as for other modes

* tests/bench-slope.c (bench_xts_encrypt_init): Use same buffer
sizes as other tests.
(bench_xts_encrypt_do_bench, bench_xts_decrypt_do_bench): Remove.
(xts_encrypt_ops): Use 'bench_encrypt_do_bench'.
(xts_decrypt_ops): Use 'bench_decrypt_do_bench'.

XTS benchmarking was not using same procedure and was not giving
comparable results to, for example, bench-slope-openssl at
'https://github.com/jkivilin/bench-slopes'

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>