Add test to enforce that program id account info for CPI is optional (#22069) (#22103)

* Update tests to demonstrate that program id account info for CPI is optional

* Clean up comments that say that program id account info is required

(cherry picked from commit ec7536faf6)

Co-authored-by: Justin Starry <justin@solana.com>

programs/bpf/rust/upgraded/src/lib.rs

@@ -8,13 +8,12 @@ use solana_program::{
 
 entrypoint!(process_instruction);
 fn process_instruction(
-    program_id: &Pubkey,
+    _program_id: &Pubkey,
     accounts: &[AccountInfo],
     _instruction_data: &[u8],
 ) -> ProgramResult {
     msg!("Upgraded program");
-    assert_eq!(accounts.len(), 2);
-    assert_eq!(accounts[0].key, program_id);
-    assert_eq!(*accounts[1].key, clock::id());
+    assert_eq!(accounts.len(), 1);
+    assert_eq!(*accounts[0].key, clock::id());
     Err(43.into())
 }