Fix intermittent test failure in test_deny_executable_write (#1588)

The test was writing 42 to offset 4 of the program account. The program
is upgradable, so offset 4 is first byte of the account which holds the
actual elf program.

There was a chance that the first byte of the account is 42, in which
case the test passes, because with the non-direct mapping case, you can
write to read only accounts as long as the contents does not change.

programs/sbf/tests/programs.rs

@@ -4451,7 +4451,7 @@ fn test_deny_executable_write() {
         ];
 
         let mut instruction_data = vec![TEST_WRITE_ACCOUNT, 2];
-        instruction_data.extend_from_slice(4usize.to_le_bytes().as_ref());
+        instruction_data.extend_from_slice(3usize.to_le_bytes().as_ref());
         instruction_data.push(42);
         let instruction = Instruction::new_with_bytes(
             invoke_program_id,