tests/t-mpi-point: add reduction test-vectors for NIST curves.

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

This is partial cherry-pick of master commit:
fc92c609dfdbcf59a09ca3aaf53a1c1b8408c351 (ec-nist: fix 'mod p' carry
adjustment and output masking).

• GnuPG-bug-id: T5510
• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>