Merge bitcoin/bitcoin#31093: Introduce g_fuzzing global for fuzzing checks

9f243cd7fa Introduce `g_fuzzing` global for fuzzing checks (dergoegge)

Pull request description:

  This PR introduces a global `g_fuzzing` that indicates if we are fuzzing.

  If `g_fuzzing` is `true` then:

  * Assume checks are enabled
  * Special fuzzing paths are taken (e.g. pow check is reduced to one bit)

  Closes #30950 #31057

ACKs for top commit:
  maflcko:
    review ACK 9f243cd7fa 🗜
  brunoerg:
    crACK 9f243cd7fa
  marcofleon:
    Tested ACK 9f243cd7fa

Tree-SHA512: 56e4cad0555dec0c565ea5ecc529628ee4f37d20dc660c647fdc6948fbeed8291e6fe290de514bd4c2c7089654d9ce1add607dc9855462828b62be9ee45e4999

src/test/fuzz/fuzz.cpp

@@ -102,6 +102,8 @@ void ResetCoverageCounters() {}
 
 void initialize()
 {
+    g_fuzzing = true;
+
     // By default, make the RNG deterministic with a fixed seed. This will affect all
     // randomness during the fuzz test, except:
     // - GetStrongRandBytes(), which is used for the creation of private key material.