runtime: TestCgoExternalThreadSignal failures on ios

[bcmills]

https://storage.googleapis.com/go-build-log/8aef7969/ios-arm64-corellium_3c768ebb.log:

--- FAIL: TestCgoExternalThreadSignal (5.15s)
    crash_cgo_test.go:113: /tmp/workdir-host-ios-arm64-corellium-ios/tmp/go-build3960206703/testprogcgo.exe CgoExternalThreadSignal: exit status 1
    crash_cgo_test.go:115: expected "OK\n", but got:
        C signal did not crash as expected
        
        
FAIL
FAIL	runtime	50.751s

(attn @golang/ios)

[gopherbot]

Change https://go.dev/cl/491095 mentions this issue: runtime: add test skips for ios