internal/testutils: skip tests if local network isn't supported

Happens in the Nix sandbox.

go.mod

@@ -8,6 +8,7 @@ require (
 	github.com/sirupsen/logrus v1.9.3
 	github.com/stretchr/testify v1.8.4
 	github.com/urfave/cli/v2 v2.25.7
+	golang.org/x/net v0.15.0
 )
 
 require (

go.sum

@@ -55,6 +55,8 @@ golang.org/x/net v0.0.0-20210226172049-e18ecbb05110/go.mod h1:m0MpNAwzfU5UDzcl9v
 golang.org/x/net v0.0.0-20210405180319-a5a99cb37ef4/go.mod h1:p54w0d4576C0XHj96bSt6lcn1PtDYWL6XObtHCRCNQM=
 golang.org/x/net v0.0.0-20220722155237-a158d28d115b/go.mod h1:XRhObCWvk6IyKnWLug+ECip1KBveYUHfp+8e9klMJ9c=
 golang.org/x/net v0.6.0/go.mod h1:2Tu9+aMcznHK/AK1HMvgo6xiTLG5rD5rZLDS+rp2Bjs=
+golang.org/x/net v0.15.0 h1:ugBLEUaxABaB5AJqW9enI0ACdci2RUd4eP51NTBvuJ8=
+golang.org/x/net v0.15.0/go.mod h1:idbUs1IY1+zTqbi8yxTbhexhEEk5ur9LInksu6HrEpk=
 golang.org/x/oauth2 v0.12.0 h1:smVPGxink+n1ZI5pkQa8y6fZT0RW0MgCO5bFpepy4B4=
 golang.org/x/oauth2 v0.12.0/go.mod h1:A74bZ3aGXgCY0qaIC9Ahg6Lglin4AMAco8cIv9baba4=
 golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=

internal/testutils.go

@@ -19,9 +19,10 @@
 package internal
 
 import (
-	"net"
 	"testing"
 
+	"golang.org/x/net/nettest"
+
 	"github.com/emersion/go-imap/backend/memory"
 	"github.com/emersion/go-imap/server"
 	"github.com/stretchr/testify/assert"
@@ -49,10 +50,10 @@ func BuildTestIMAPServer(t *testing.T) (*server.Server, string, *memory.Mailbox)
 
 	s.AllowInsecureAuth = true
 
-	l, err := net.Listen("tcp", "localhost:0")
-	assert.NoError(t, err)
+	l, err := nettest.NewLocalListener("tcp")
 	if err != nil {
-		t.FailNow()
+		t.Logf("%v", err)
+		t.Skip()
 	}
 
 	go func() { err = s.Serve(l) }()