fuzz_tests: set small subaddress lookahead for speed

tests/fuzz/cold-outputs.cpp

@@ -54,6 +54,7 @@ int ColdOutputsFuzzer::init()
   try
   {
     wallet.init("");
+    wallet.set_subaddress_lookahead(1, 1);
     wallet.generate("", "", spendkey, true, false);
   }
   catch (const std::exception &e)

tests/fuzz/cold-transaction.cpp

@@ -55,6 +55,7 @@ int ColdTransactionFuzzer::init()
   try
   {
     wallet.init("");
+    wallet.set_subaddress_lookahead(1, 1);
     wallet.generate("", "", spendkey, true, false);
   }
   catch (const std::exception &e)

tests/fuzz/signature.cpp

@@ -55,6 +55,7 @@ int SignatureFuzzer::init()
   try
   {
     wallet.init("");
+    wallet.set_subaddress_lookahead(1, 1);
     wallet.generate("", "", spendkey, true, false);
 
     cryptonote::address_parse_info info;