tests: disable mknod/mknodat on openbsd too (EPERM errno). #1699

Job	Run time
macos-aarch64	1m 54s
linux_native_builds (x86_64-unknown-linux-gnu)	2m 38s
macos	3m 8s
linux_native_builds (x86_64-unknown-linux-musl)	2m 29s
rust_stable	1m 21s
minver	14s
rustfmt	5s
solaris (x86_64-pc-solaris)	4m 17s
tier3 (x86_64-unknown-dragonfly)	1m 1s
tier3 (x86_64-unknown-openbsd)	1m 1s
tier3 (x86_64-unknown-haiku)	1m 1s
tier3 (armv7-unknown-linux-uclibceabihf)	1m 9s
redox	37s
cross_compiling (aarch64-linux-android)	36s
cross (arm-unknown-linux-gnueabi)	1m 54s
cross_compiling (arm-linux-androideabi)	35s
cross (armv7-unknown-linux-gnueabihf)	1m 56s
cross_compiling (armv7-linux-androideabi)	35s
cross (i686-unknown-linux-gnu)	2m 1s
cross_compiling (i686-linux-android)	41s
cross (i686-unknown-linux-musl)	2m 1s
cross_compiling (x86_64-linux-android)	39s
cross (mips-unknown-linux-gnu)	1m 52s
cross_compiling (arm-unknown-linux-musleabi)	37s
cross (mips64-unknown-linux-gnuabi64)	2m 1s
cross_compiling (x86_64-unknown-fuchsia)	31s
cross (mips64el-unknown-linux-gnuabi64)	2m 9s
cross_compiling (x86_64-unknown-illumos)	34s
cross_compiling (aarch64-apple-ios, true)	32s
cross (mipsel-unknown-linux-gnu)	1m 48s
cross_compiling (powerpc64-unknown-linux-gnu)	44s
cross (powerpc64le-unknown-linux-gnu)	1m 53s
cross_compiling (s390x-unknown-linux-gnu)	43s
cross (loongarch64-unknown-linux-gnu)	2m 6s
cross_compiling (x86_64-unknown-linux-gnux32)	42s
cross_compiling (x86_64-unknown-netbsd)	31s
	48m 36s

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