Using flag breaks tests

[andy-trimble]

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

$ go version
go version go1.13 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/andrewtrimble/Library/Caches/go-build"
GOENV="/Users/andrewtrimble/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/andrewtrimble/Workspace/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.13/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.13/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
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/bg/m0hrtm6s7v9drh13khyj29t80000gn/T/go-build368297642=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

main.go

package main

import (
	"flag"
	"fmt"
)

var (
	fl = flag.Bool("fl", false, "")
)

func init() {
	flag.Parse()
}

func main() {
	fmt.Printf("%t\n", *fl)
}

main_test.go

package main

import "testing"

func TestBug(t *testing.T) {

}

What did you expect to see?

go test
PASS
ok      github.com/andy-trimble/flag-bug        0.005s

What did you see instead?

go test
flag provided but not defined: -test.timeout
Usage of /var/folders/bg/m0hrtm6s7v9drh13khyj29t80000gn/T/go-build828887410/b001/flag-bug.test:
  -fl
    
exit status 2
FAIL    github.com/andy-trimble/flag-bug        0.005s

[andy-trimble]

This is similar to #31859.

[ALTree]

This is expected. From the 1.13 release notes:

Testing flags are now registered in the new Init function, which is invoked by the generated main function for the test. As a result, testing flags are now only registered when running a test binary, and packages that call flag.Parse during package initialization may cause tests to fail.

https://tip.golang.org/doc/go1.13#testing

Closing here as a dup of #31859