net: TestLookupCNAME failing on Plan 9

[0intro]

Since CL 34650, TestLookupCNAME is failing on Plan 9.

--- FAIL: TestLookupCNAME (0.05s)
	lookup_test.go:266: write /net/dns: dns: dns failure
FAIL
FAIL	net	22.602s

See https://build.golang.org/log/e4a9e619f8eff94f133e8748c02c1acc7b2ddfe1

[gopherbot]

CL https://golang.org/cl/34633 mentions this issue.