Fix grpc test vet warning (#3341)
Fix grpc test vet warning (#3341)

This fixes the vet warning: the cancel function returned by
context.WithTimeout should be called, not discarded, to avoid a context
leak.

Signed-off-by: Ingo Gottwald <b3a546640b0c7ce51856389753755060a3dadcdd@gmail.com>
miekg/coredns,chrisohaver/coredns,yongtang/coredns,miekg/coredns,johnbelamaric/coredns,coredns/coredns,johnbelamaric/coredns