Fix test_signal test for AIX #2106

Job	Run time
macos-aarch64	4m 26s
aarch64_linux_native_builds (aarch64-unknown-linux-gnu)	4m 16s
x86_64_linux_native_builds (x86_64-unknown-linux-gnu)	4m 18s
aarch64_linux_native_builds (aarch64-unknown-linux-musl)	4m 14s
rust_stable	1m 33s
macos	9m 27s
tier3 (armv7-unknown-linux-uclibceabihf)	53s
tier3 (x86_64-unknown-haiku)	43s
tier3 (x86_64-unknown-openbsd)	44s
x86_64_linux_native_builds (x86_64-unknown-linux-musl)	4m 12s
tier3 (x86_64-unknown-dragonfly)	46s
minver	20s
solaris (x86_64-pc-solaris)	6m 12s
rustfmt	14s
cross_compiling	0s
cross	0s
redox	0s
	42m 18s

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