[gccgo] skip # gccgo has no cover tool

! go test -n importmain/test
stderr 'not an importable package' # check that import main was detected
! go test -n -cover importmain/test
stderr 'not an importable package' # check that import main was detected

-- go.mod --
module importmain

go 1.16
-- ismain/main.go --
package main

import _ "importmain/test"

func main() {}
-- test/test.go --
package test
-- test/test_test.go --
package test_test

import "testing"
import _ "importmain/ismain"

func TestCase(t *testing.T) {}