p256: use `primefield::test_primefield` to test `FieldElement` (#1322) #77

Job	Run time
test (1.85.0)	31s
test (stable)	40s
build (1.85.0, thumbv7em-none-eabi)	24s
build (1.85.0, wasm32-unknown-unknown)	26s
build (stable, thumbv7em-none-eabi)	22s
doc	39s
build (stable, wasm32-unknown-unknown)	25s
	3m 27s

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