p521: projective arithmetic tests #1776

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	6m 24s
doc	37s
cross (armv7-unknown-linux-gnueabihf, 1.65.0)	8m 9s
build (1.65.0, wasm32-unknown-unknown)	6m 10s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 31s
build (stable, thumbv7em-none-eabi)	35s
cross (aarch64-unknown-linux-gnu, 1.65.0)	4m 22s
build (stable, wasm32-unknown-unknown)	1m 20s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	7m 37s
cross (aarch64-unknown-linux-gnu, stable)	2m 1s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	3m 15s
cross (powerpc-unknown-linux-gnu, 1.65.0)	4m 36s
test (x86_64-unknown-linux-gnu, 1.65.0)	6m 52s
cross (powerpc-unknown-linux-gnu, stable)	2m 38s
test (x86_64-unknown-linux-gnu, stable)	2m 4s
	58m 11s

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