Tests

Make getrandom a non-pub dep; rename feature to os_rng #1180

Job	Run time
test-ios	26s
Check doc	27s
test-cross (ubuntu-latest, powerpc-unknown-linux-gnu, stable)	1m 4s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	13s
test-ks	37s
test (macos-latest, x86_64-apple-darwin, stable)	19s
test-miri	3m 55s
test-no-std	13s
test (windows-latest, x86_64-pc-windows-gnu, stable)	2m 40s
test (windows-latest, x86_64-pc-windows-msvc, beta)	1m 14s
test (ubuntu-latest, x86_64-unknown-linux-gnu, MSRV, 1.61.0)	1m 59s
test (ubuntu-latest, sudo apt-get update ; sudo apt install gcc-multilib, i686-unknown-linux-gnu,...	1m 12s
test (ubuntu-latest, x86_64-unknown-linux-gnu, nightly, minimal_versions)	1m 0s
	15m 19s

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