tests/bench-slope: add missing cipher context reset

* tests/bench-slope.c (bench_encrypt_do_bench)
(bench_decrypt_do_bench): Add call to 'gcry_cipher_reset'.

Some non-AEAD results were negativily affected by missing state
reset (~1% for aesni-ctr and chacha20-stream).

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