p256

Fix test vectors for bign256 crate #2072

Job	Run time
build (1.73.0, thumbv7em-none-eabi)	44s
build (1.73.0, wasm32-unknown-unknown)	36s
cross (armv7-unknown-linux-gnueabihf, 1.73.0)	1m 34s
build (stable, thumbv7em-none-eabi)	40s
doc	59s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 34s
build (stable, wasm32-unknown-unknown)	35s
cross (aarch64-unknown-linux-gnu, 1.73.0)	1m 28s
cross (aarch64-unknown-linux-gnu, stable)	1m 17s
test (i686-unknown-linux-gnu, 1.73.0, sudo apt update && sudo apt install gcc-multilib)	1m 50s
cross (powerpc-unknown-linux-gnu, 1.73.0)	1m 35s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 53s
cross (powerpc-unknown-linux-gnu, stable)	1m 22s
test (x86_64-unknown-linux-gnu, 1.73.0)	1m 29s
test (x86_64-unknown-linux-gnu, stable)	1m 25s
	19m 1s

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