fix(netxlite): clearly document quirk and make code robust (#468)

This quirk really saddens me. It's a piece of tech debt we're
carrying over from the original netx implementation.

We cannot remove it _until_ we have legacy netx code around.

The second best thing we can do is to clearly move this code in
a place where it's clear it's a quirk and write and use some extra
code that makes sure the quirk's assumptions are always met.

Sigh.

See https://github.com/ooni/probe/issues/1591

internal/netxlite/legacy_test.go

@@ -1,52 +1,12 @@
 package netxlite
 
 import (
-	"errors"
 	"net"
 	"testing"
 
-	"github.com/ooni/probe-cli/v3/internal/errorsx"
 	"github.com/ooni/probe-cli/v3/internal/netxlite/mocks"
 )
 
-func TestReduceErrors(t *testing.T) {
-	t.Run("no errors", func(t *testing.T) {
-		result := reduceErrors(nil)
-		if result != nil {
-			t.Fatal("wrong result")
-		}
-	})
-	t.Run("single error", func(t *testing.T) {
-		err := errors.New("mocked error")
-		result := reduceErrors([]error{err})
-		if result != err {
-			t.Fatal("wrong result")
-		}
-	})
-	t.Run("multiple errors", func(t *testing.T) {
-		err1 := errors.New("mocked error #1")
-		err2 := errors.New("mocked error #2")
-		result := reduceErrors([]error{err1, err2})
-		if result.Error() != "mocked error #1" {
-			t.Fatal("wrong result")
-		}
-	})
-	t.Run("multiple errors with meaningful ones", func(t *testing.T) {
-		err1 := errors.New("mocked error #1")
-		err2 := &errorsx.ErrWrapper{
-			Failure: "unknown_failure: antani",
-		}
-		err3 := &errorsx.ErrWrapper{
-			Failure: errorsx.FailureConnectionRefused,
-		}
-		err4 := errors.New("mocked error #3")
-		result := reduceErrors([]error{err1, err2, err3, err4})
-		if result.Error() != errorsx.FailureConnectionRefused {
-			t.Fatal("wrong result")
-		}
-	})
-}
-
 func TestResolverLegacyAdapterWithCompatibleType(t *testing.T) {
 	var called bool
 	r := NewResolverLegacyAdapter(&mocks.Resolver{