Add tests for handling Offers with fiat amount

Ensure successful creation of InvoiceRequest when a
currency amount is specified without amount_msats. Then
ideally fail when both currency amount and amount_msats
are provided. However, the return behavior isn't what's
expected; its a work in progress.

lightning/src/offers/invoice_request.rs

@@ -1721,6 +1721,24 @@ mod tests {
 			Ok(_) => panic!("expected error"),
 			Err(e) => assert_eq!(e, Bolt12SemanticError::InvalidAmount),
 		}
+
+		let invoice_request = OfferBuilder::new(recipient_pubkey())
+			.amount(Amount::Currency {iso4217_code: *b"USD", amount: 1000})
+			.build_unchecked()
+			.request_invoice(vec![1; 32], payer_pubkey()).unwrap()
+			.build().unwrap()
+			.sign(payer_sign).unwrap();
+		assert_eq!(invoice_request.amount(), Some(Amount::Currency {iso4217_code: *b"USD", amount: 1000}));
+
+		match OfferBuilder::new(recipient_pubkey())
+			.amount(Amount::Currency {iso4217_code: *b"USD", amount: 100})
+			.build_unchecked()
+			.request_invoice(vec![1; 32], payer_pubkey())
+			.unwrap().amount_msats(150_000_000)
+		{
+			Ok(_) => panic!("expected error"),
+			Err(e) => assert_eq!(e, Bolt12SemanticError::UnsupportedCurrency),
+		}
 	}
 
 	#[test]