os/exec: tests doesn't build on Plan 9

[0intro]

Since CL 36800, the os/exec tests doesn't build anymore on Plan 9.
It happens because poll.PollDescriptor is not defined on Plan 9.

# os/exec_test
go/src/os/exec/exec_test.go:380: undefined: poll.PollDescriptor
go/src/os/exec/exec_test.go:740: undefined: poll.PollDescriptor
FAIL	os/exec [build failed]

See https://build.golang.org/log/8dad14ea8ba744cda507490c6267a7665c6021d7

[ianlancetaylor]

CL 37106.

[gopherbot]

CL https://golang.org/cl/37107 mentions this issue.

[gopherbot]

CL https://golang.org/cl/37106 mentions this issue.