Add benchmark for IsCached.

Thanks to Cecylia for questioning whether the cache lookup mechanism is
fast enough:
ecc6e048 (comment 2709958)

tor_test.go

@@ -2,8 +2,10 @@ package main
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"io/ioutil"
+	"math/rand"
 	"net"
 	"os"
 	"testing"
@@ -107,6 +109,30 @@ func TestCacheExpiration(t *testing.T) {
 	}
 }
 
+func BenchmarkIsCached(b *testing.B) {
+
+	getRandAddrPort := func() string {
+		return fmt.Sprintf("%d.%d.%d.%d:%d",
+			rand.Intn(256), rand.Intn(256), rand.Intn(256), rand.Intn(256), rand.Intn(65536))
+	}
+	getRandError := func() error {
+		errors := []error{nil, errors.New("censorship"), errors.New("no censorship")}
+		return errors[rand.Intn(len(errors))]
+	}
+
+	numCacheEntries := 10000
+	cache := make(TestCache)
+	for i := 0; i < numCacheEntries; i++ {
+		cache.AddEntry(getRandAddrPort(), getRandError())
+	}
+
+	// How long does it take to iterate over numCacheEntries cache entries?
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		cache.IsCached("invalid bridge line")
+	}
+}
+
 func TestCacheSerialisation(t *testing.T) {
 
 	cache := make(TestCache)