Bignum tests: add arity
Bignum tests: add arity

Add the ability to control the number of operands, by setting the arity
class attribute.

Signed-off-by: Janos Follath <b175c17abe0474719672be1367bb75ee28bd5c71@arm.com>

Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls