Enable caller to select small initial nonce

... for reproducing quite closely behaviour before 68124a3

src/aeminer_pow.erl

@@ -9,6 +9,7 @@
 -export([integer_to_scientific/1,
          next_nonce/2,
          pick_nonce/0,
+         pick_nonce/1,
          scientific_to_integer/1,
          target_to_difficulty/1,
          test_target/2,
@@ -118,7 +119,11 @@ next_nonce(Nonce, Cfg) ->
 
 -spec pick_nonce() -> nonce().
 pick_nonce() ->
-    rand:uniform(?MAX_NONCE + 1) - 1.
+    pick_nonce(?MAX_NONCE).
+
+-spec pick_nonce(1..?MAX_NONCE) -> nonce().
+pick_nonce(Max) ->
+    rand:uniform(Max + 1) - 1.
 
 -spec trim_nonce(nonce(), config()) -> nonce().
 trim_nonce(Nonce, Cfg) ->