Tests

Make getrandom a non-pub dep; rename feature to os_rng #1185

Job	Run time
test-cross (ubuntu-latest, powerpc-unknown-linux-gnu, stable)	1m 10s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	57s
test-ks	36s
test (macos-latest, x86_64-apple-darwin, stable)	52s
Check doc	32s
test (windows-latest, x86_64-pc-windows-gnu, stable)	3m 25s
test-ios	22s
test (windows-latest, x86_64-pc-windows-msvc, beta)	2m 0s
test (ubuntu-latest, x86_64-unknown-linux-gnu, MSRV, 1.63.0)	2m 57s
test-miri	4m 32s
test (ubuntu-latest, sudo apt-get update ; sudo apt install gcc-multilib, i686-unknown-linux-gnu,...	1m 47s
test-no-std	13s
test (ubuntu-latest, x86_64-unknown-linux-gnu, nightly, minimal_versions)	1m 25s
	20m 48s

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