testing: benchmark example shows vet warning

[178inaba]

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

$ go version
go version go1.13.4 darwin/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/masahiro.furudate/Library/Caches/go-build"
GOENV="/Users/masahiro.furudate/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/masahiro.furudate/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.13.4/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.13.4/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/masahiro.furudate/work/go/go/src/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/wp/wd7n1rkd5ln8v4c3qsw891vnkvjmym/T/go-build202688006=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I ran go vet on the very first Benchmarks example.
https://golang.org/pkg/testing/#hdr-Benchmarks

What did you expect to see?

No warning.

What did you see instead?

I saw the warning "result of fmt.Sprintf call not used".

[gopherbot]

Change https://golang.org/cl/220397 mentions this issue: testing: fix vet warning in example code