os/exec: disable fd check in TestHelperProcess on Plan 9
os/exec: disable fd check in TestHelperProcess on Plan 9

On Plan 9, we can observe the following open file descriptors:

  0 r  c    0 (000000000000000a   0 00)     0        0 /dev/null
  1 rw |    0 (0000000001df6742   0 00) 65536       54 #|/data1
  2 rw |    0 (0000000001df6782   0 00) 65536        0 #|/data1
  3 rw M 1956 (0000000000d66dd2   0 00)  8192       12 /tmp/333163398
  4 r  c    0 (0000000000000001   0 00)     0      528 /dev/bintime
  5 r  M 1956 (0000000000d66dd1 854 00)  8192        0 /tmp/go-build843954301/os/exec/_test/exec.test
  6 r  M 1956 (0000000000d66dd1 854 00)  8192        0 /tmp/go-build843954301/os/exec/_test/exec.test
  7 r  M 1956 (0000000000d66dd1 854 00)  8192        0 /tmp/go-build843954301/os/exec/_test/exec.test
  8 r  M 1956 (0000000000d66dd1 854 00)  8192        0 /tmp/go-build843954301/os/exec/_test/exec.test
  9 r  M 1956 (0000000000d66dd1 854 00)  8192        0 /tmp/go-build843954301/os/exec/_test/exec.test
 10 r  M 1956 (0000000000d66dd1 854 00)  8192        0 /tmp/go-build843954301/os/exec/_test/exec.test
 11 r  c    0 (000000000000000f   0 00)     0       32 /dev/random
 12 r  M 1956 (0000000000d66dd1 854 00)  8192        0 /tmp/go-build843954301/os/exec/_test/exec.test
 13 r  c    0 (000000000000000a   0 00)     0        0 /dev/null
 14 rw |    0 (0000000001df6801   0 00) 65536        0 #|/data
 15 rw |    0 (0000000001df6802   0 00) 65536     1275 #|/data1

R=rsc, bradfitz, aram
CC=golang-codereviews
https://codereview.appspot.com/51420044

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang