x/sys/unix: TestPpoll and TestPoll fail a `test all`

[hdonnay]

What version of Go are you using (go version)?

go version go1.11 linux/amd64

What did you do?

cd $(mktemp -d)
cat >ex.go <<.
package main

import (
	"fmt"

	_ "golang.org/x/sys/unix"
)

func main() {
	fmt.Println("example")
}

.
go mod init example && go test all; echo $?

What did you expect to see?

0

What did you see instead?

1

The tests fail, presumably because it's run in a non-writable place.

--- FAIL: TestPpoll (0.00s)
    syscall_unix_test.go:613: mktmpfifo: failed to create FIFO: permission denied
--- FAIL: TestPoll (0.00s)
    syscall_unix_test.go:613: mktmpfifo: failed to create FIFO: permission denied
FAIL
FAIL	golang.org/x/sys/unix	0.171s

With modules, is there a place that tests can write to?

[tklauser]

/cc @bradfitz @ianlancetaylor

[bcmills]

Fixed by CL 162987.