srp: convert tests to proptests (#284) #294

Job	Run time
build (stable, thumbv7em-none-eabi)	18s
test (1.85)	2m 25s
test (stable)	2m 16s
build (1.85, thumbv7em-none-eabi)	17s
build (stable, wasm32-unknown-unknown)	17s
build (1.85, wasm32-unknown-unknown)	18s
	5m 51s

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