Tests

remove conditional compilation around using core #1173

Job	Run time
test-ios	30s
test-cross (ubuntu-latest, powerpc-unknown-linux-gnu, stable)	1m 17s
Check doc	33s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	1m 8s
test-ks	37s
test (macos-latest, x86_64-apple-darwin, stable)	53s
test-miri	4m 38s
test (windows-latest, x86_64-pc-windows-gnu, stable)	3m 35s
test-no-std	14s
test (windows-latest, x86_64-pc-windows-msvc, beta)	1m 56s
test (ubuntu-latest, x86_64-unknown-linux-gnu, MSRV, 1.61.0)	2m 59s
test (ubuntu-latest, sudo apt-get update ; sudo apt install gcc-multilib, i686-unknown-linux-gnu,...	1m 54s
test (ubuntu-latest, x86_64-unknown-linux-gnu, nightly, minimal_versions)	1m 25s
	21m 39s

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