cp f.go $WORK/tmp/testdata/src/xtestonly/f.go
cp f_test.go $WORK/tmp/testdata/src/xtestonly/f_test.go
env GOPATH=$WORK/tmp/testdata
cd $WORK

# Check output of go list to ensure no duplicates
go list xtestonly ./tmp/testdata/src/xtestonly/...
cmp stdout $WORK/gopath/src/wantstdout

-- wantstdout --
xtestonly
-- f.go --
package xtestonly

func F() int { return 42 }
-- f_test.go --
package xtestonly_test

import (

[short] skip

go test xtestonly
! stdout '^ok.*\[no tests to run\]'
stdout '^ok'

-- go.mod --
module xtestonly

go 1.16
-- f.go --
package xtestonly

func F() int { return 42 }
-- f_test.go --
package xtestonly_test

import (
	"testing"
	"xtestonly"
)

func TestF(t *testing.T) {
	if x := xtestonly.F(); x != 42 {
		t.Errorf("f.F() = %d, want 42", x)
	}