hooks/test: fix incorrect use of math/rand

Fix incorrect uses of math/rand:
- do not call rand.Seed() in a test as it affects the global pseudo-random number
  generator and might affect other tests (or Logrus itself). Instead, use a local
  instance of a number generator
- seed with time.Now().UnixNano() instead of time.Now().Unix() for more
  randomness
- use "rand.Intn(100)" instead of "rand.Int() % 100"

hooks/test/test_test.go

@@ -41,8 +41,8 @@ func TestAllHooks(t *testing.T) {
 
 func TestLoggingWithHooksRace(t *testing.T) {
 
-	rand.Seed(time.Now().Unix())
-	unlocker := rand.Int() % 100
+	r := rand.New(rand.NewSource(time.Now().UnixNano()))
+	unlocker := r.Intn(100)
 
 	assert := assert.New(t)
 	logger, hook := NewNullLogger()