Combine test_output_substitution with sender checklist test for clarity (payjoin#767)

By combining the `test_output_substitution` test and
`test_process_proposal_when_payee_output_has_disallowed_output_substitution`
we can de-duplicate some test logic and ensure that these tests cover
all of the checklist test vectors.

Hopefully we can get all the checks completed in our
[checklist](payjoin#339 (comment))
with these.

Author: spacebear21

payjoin/src/send/mod.rs

@@ -719,29 +719,6 @@ mod test {
         Ok(())
     }
 
-    #[test]
-    fn test_output_substitution() -> Result<(), BoxError> {
-        let mut ctx = create_psbt_context()?;
-        let mut proposal = PARSED_PAYJOIN_PROPOSAL.clone();
-
-        ctx.output_substitution = OutputSubstitution::Disabled;
-        assert!(ctx.clone().process_proposal(proposal.clone()).is_ok(),);
-
-        std::mem::swap(
-            &mut ctx.original_psbt.unsigned_tx.output[0].value,
-            &mut proposal.unsigned_tx.output[0].value,
-        );
-
-        ctx.original_psbt.unsigned_tx.output[0].script_pubkey = ctx.payee.clone();
-
-        assert!(matches!(
-            ctx.clone().process_proposal(proposal.clone()).unwrap_err(),
-            InternalProposalError::DisallowedOutputSubstitution
-        ));
-
-        Ok(())
-    }
-
     #[test]
     fn test_payee_output_value_decreased() -> Result<(), BoxError> {
         let mut ctx = create_psbt_context()?;
@@ -1118,12 +1095,23 @@ mod test {
         fn test_process_proposal_when_payee_output_has_disallowed_output_substitution(
         ) -> Result<(), BoxError> {
             let mut ctx = create_psbt_context()?;
-            let proposal = PARSED_PAYJOIN_PROPOSAL.clone();
+            let mut proposal = PARSED_PAYJOIN_PROPOSAL.clone();
+            ctx.output_substitution = OutputSubstitution::Disabled;
+
+            // When output substitution is disabled ensure that the output value did not decrease
+            assert!(ctx.clone().process_proposal(proposal.clone()).is_ok());
+
+            // When output substitution is disabled still allow increasing the output value
+            proposal.unsigned_tx.output[0].value += Amount::from_sat(182);
+            assert!(ctx.clone().process_proposal(proposal.clone()).is_ok());
 
+            proposal.unsigned_tx.output[0].value -= Amount::from_sat(182);
             ctx.original_psbt.unsigned_tx.output.get_mut(0).unwrap().script_pubkey =
                 ctx.payee.clone();
-            ctx.output_substitution = OutputSubstitution::Disabled;
-
+            std::mem::swap(
+                &mut ctx.original_psbt.unsigned_tx.output[0].value,
+                &mut proposal.unsigned_tx.output[0].value,
+            );
             assert_eq!(
                 ctx.process_proposal(proposal).unwrap_err().to_string(),
                 InternalProposalError::DisallowedOutputSubstitution.to_string()
@@ -1132,6 +1120,27 @@ mod test {
             Ok(())
         }
 
+        #[test]
+        fn test_process_proposal_when_payee_output_has_allowed_output_substitution(
+        ) -> Result<(), BoxError> {
+            let mut ctx = create_psbt_context()?;
+            let mut proposal = PARSED_PAYJOIN_PROPOSAL.clone();
+
+            // Do not make any checks when output substitution is enabled
+            ctx.output_substitution = OutputSubstitution::Enabled;
+            ctx.original_psbt.unsigned_tx.output.get_mut(0).unwrap().script_pubkey =
+                ctx.payee.clone();
+            assert!(ctx.clone().process_proposal(proposal.clone()).is_ok());
+
+            proposal.unsigned_tx.output[0].value += Amount::from_sat(182);
+            assert!(ctx.clone().process_proposal(proposal.clone()).is_ok());
+
+            proposal.unsigned_tx.output[0].value -= Amount::from_sat(364);
+            assert!(ctx.process_proposal(proposal).is_ok());
+
+            Ok(())
+        }
+
         #[test]
         fn test_process_proposal_when_output_value_decreased() -> Result<(), BoxError> {
             let mut ctx = create_psbt_context()?;