hooks/test: fix incorrect use of math/rand

[dolmen]

Fix incorrect uses of math/rand:

• do not call rand.Seed() (deprecated since Go 1.20) 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