tests: add root_tests (tests that must run as root)

First test is a reproducer for
https://github.com/rfjakob/gocryptfs/issues/394 "Group permissions: No write access"

Makefile

@@ -7,6 +7,11 @@ build:
 test:
 	./test.bash
 
+.phony: root_test
+root_test:
+	./build.bash
+	cd tests/root_test ; go test -c ; sudo ./root_test.test
+
 .phony: format
 format:
 	go fmt ./...

tests/root_test/root_test.go

@@ -0,0 +1,60 @@
+package root_test
+
+import (
+	"os"
+	"runtime"
+	"syscall"
+	"testing"
+
+	"github.com/rfjakob/gocryptfs/tests/test_helpers"
+)
+
+func asUser(uid int, gid int, supplementaryGroups []int, f func() error) error {
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	err := syscall.Setgroups(supplementaryGroups)
+	if err != nil {
+		return err
+	}
+	defer syscall.Setgroups(nil)
+
+	err = syscall.Setregid(-1, gid)
+	if err != nil {
+		return err
+	}
+	defer syscall.Setregid(-1, 0)
+
+	err = syscall.Setreuid(-1, uid)
+	if err != nil {
+		return err
+	}
+	defer syscall.Setreuid(-1, 0)
+
+	return f()
+}
+
+func TestSupplementaryGroups(t *testing.T) {
+	if os.Getuid() != 0 {
+		t.Skip("must run as root")
+	}
+	cDir := test_helpers.InitFS(t)
+	pDir := cDir + ".mnt"
+	test_helpers.MountOrFatal(t, cDir, pDir, "-allow_other", "-extpass=echo test")
+	defer test_helpers.UnmountPanic(pDir)
+
+	dir1 := pDir + "/dir1"
+	err := os.Mkdir(dir1, 0770)
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = os.Chown(dir1, 0, 1234)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = asUser(1235, 1235, []int{1234}, func() error { return os.Mkdir(dir1+"/foo", 0700) })
+	if err != nil {
+		t.Error(err)
+	}
+}