tests/t-mpi-point: add NIST curve multiplication test vectors

* tests/t-mpi-point.c (mpi_base10_scan, check_ec_mul): New.
(main): Call 'check_ec_mul'.

These vectors are from: http://point-at-infinity.org/ecc/nisttv

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