Add #[allow(unused_mut)] to fix compilation on latest nightly

The current stable performs an unecessary mutable borrow of these
variables, so we can't just remove the 'mut'

Author: Aaron1011

programs/bpf_loader/src/lib.rs

@@ -401,6 +401,7 @@ mod tests {
         );
 
         // Case: Write bytes to an offset
+        #[allow(unused_mut)]
         let mut keyed_accounts = vec![KeyedAccount::new(&program_key, true, &program_account)];
         keyed_accounts[0].account.borrow_mut().data = vec![0; 6];
         assert_eq!(
@@ -418,6 +419,7 @@ mod tests {
         );
 
         // Case: Overflow
+        #[allow(unused_mut)]
         let mut keyed_accounts = vec![KeyedAccount::new(&program_key, true, &program_account)];
         keyed_accounts[0].account.borrow_mut().data = vec![0; 5];
         assert_eq!(

runtime/src/message_processor.rs

@@ -1354,6 +1354,7 @@ mod tests {
         let not_owned_account = Account::new(84, 1, &Pubkey::new_rand());
         let not_owned_preaccount = PreAccount::new(&not_owned_key, &not_owned_account, false, true);
 
+        #[allow(unused_mut)]
         let mut accounts = vec![
             Rc::new(RefCell::new(owned_account)),
             Rc::new(RefCell::new(not_owned_account)),