Test types MIN/MAX instead of i64::MIN/i64::MAX

The MIN/MAX for SignedAmount recently changed from i64::MIN and
i64::MAX to MAX_MONEY/MIN_MONEY.  Update the tests to reflect this new
MIN/MAX since it is no longer valid to create a value above or bellow
MAX_MONEY/MIN_MONEY.

units/src/amount/tests.rs

@@ -624,12 +624,12 @@ fn to_from_string_in() {
     assert_eq!(sa_str(&sa_sat(-1).to_string_in(D::MicroBitcoin), D::MicroBitcoin), Ok(sa_sat(-1)));
 
     assert_eq!(
-        sa_str(&sa_sat(i64::MAX).to_string_in(D::Satoshi), D::MicroBitcoin),
+        sa_str(&SignedAmount::MAX.to_string_in(D::Satoshi), D::MicroBitcoin),
         Err(OutOfRangeError::too_big(true).into())
     );
     // Test an overflow bug in `abs()`
     assert_eq!(
-        sa_str(&sa_sat(i64::MIN).to_string_in(D::Satoshi), D::MicroBitcoin),
+        sa_str(&SignedAmount::MIN.to_string_in(D::Satoshi), D::MicroBitcoin),
         Err(OutOfRangeError::too_small().into())
     );
 }
@@ -903,12 +903,12 @@ fn checked_sum_amounts() {
     assert_eq!(None, sum);
 
     let amounts =
-        [SignedAmount::from_sat(i64::MIN), SignedAmount::from_sat(-1), SignedAmount::from_sat(21)];
+        [SignedAmount::MIN, SignedAmount::from_sat(-1), SignedAmount::from_sat(21)];
     let sum = amounts.into_iter().checked_sum();
     assert_eq!(None, sum);
 
     let amounts =
-        [SignedAmount::from_sat(i64::MAX), SignedAmount::from_sat(1), SignedAmount::from_sat(21)];
+        [SignedAmount::MAX, SignedAmount::from_sat(1), SignedAmount::from_sat(21)];
     let sum = amounts.into_iter().checked_sum();
     assert_eq!(None, sum);