cd `mktemp -d`
go mod -init -module=example.com/test
cat <<EOD > main_test.go
package main

import "testing"

func Test(t *testing.T) {}
EOD
go test all

$ go test all
ok  	example.com/test	(cached)
ok  	testing	(cached)
ok  	bytes	2.515s
ok  	errors	(cached)
ok  	flag	0.037s
ok  	fmt	(cached)
?   	internal/race	[no test files]
ok  	io	(cached)
^Csignal: interrupt