Point the unreachable-app test at port 1 (closes #51)

TestAnswers502WhenTheAppCannotBeReached closed a listener and pointed
smallwebwaf at its port, which another test running in parallel could
open in between, so the test sometimes got that server's answer instead
of a 502. It now uses 127.0.0.1:1: nothing listens there, and a test
listening on port 0 is always given a port from 32768 up, so no test
can take it. No other test reuses a closed listener's port.

Model: opus-5-5

internal/proxy/passthrough_test.go

@@ -5,7 +5,6 @@ import (
 	"bytes"
 	"errors"
 	"io"
-	"net"
 	"net/http"
 	"slices"
 	"strings"
@@ -354,15 +353,8 @@ func TestRefusesHeadersOver32KiB(t *testing.T) {
 func TestAnswers502WhenTheAppCannotBeReached(t *testing.T) {
 	t.Parallel()
 
-	listener, err := (&net.ListenConfig{}).Listen(t.Context(), "tcp", localhost+":0")
-	if err != nil {
-		t.Fatalf("listen: %v", err)
-	}
-
-	closedAddr := listener.Addr().String()
-	_ = listener.Close()
-
-	addr, out := startProxy(t, "http://"+closedAddr, nil)
+	// No test can listen on port 1: listening on port 0 gets one from 32768 up.
+	addr, out := startProxy(t, "http://"+localhost+":1", nil)
 
 	wantStatus(t, get(t, addr, "/"), http.StatusBadGateway)
 	wantLine(t, out.requestLine(t), http.StatusBadGateway,