add balance transfer tests

Add tests to show the error types of different impossible balance transfers.

Since it's only tests describing the current behavior, I merge this without review. We can change the behavior in an other MR.

closes #135 (closed)

runtime/gdev/tests/balance_tests.rs

@@ -25,6 +25,7 @@ use sp_keyring::AccountKeyring;
 
 /// test currency transfer
 /// (does not take fees into account because it's only calls, not extrinsics)
+/// test final balance of Alice and Eve accounts
 #[test]
 fn test_transfer() {
     ExtBuilder::new(1, 3, 4)
@@ -52,23 +53,7 @@ fn test_transfer() {
         })
 }
 
-/// test balance transfer
-#[test]
-fn test_transfer() {
-    ExtBuilder::new(1, 3, 4)
-        .with_initial_balances(vec![(AccountKeyring::Alice.to_account_id(), 600)])
-        .build()
-        .execute_with(|| {
-            run_to_block(1);
-            assert_ok!(Balances::transfer_allow_death(
-                frame_system::RawOrigin::Signed(AccountKeyring::Alice.to_account_id()).into(),
-                AccountKeyring::Dave.to_account_id().into(),
-                500
-            ));
-        })
-}
-
-/// test balance transfer without enough balance (would prefer TokenError::BelowMinimum or NotExpendable)
+/// test balance transfer without enough balance
 #[test]
 fn test_transfer_not_enough() {
     ExtBuilder::new(1, 3, 4)
@@ -82,24 +67,26 @@ fn test_transfer_not_enough() {
                     AccountKeyring::Dave.to_account_id().into(),
                     500
                 ),
-                sp_runtime::TokenError::Frozen
+                sp_runtime::TokenError::Frozen // frozen because trying to transfer more than liquid
             );
         })
 }
 
-/// test balance transfer without enough balance (would prefer TokenError::FundsUnavailable)
+/// test balance transfer without enough balance
+/// in this case, it is the total issuance which cause problem, hence the arithmetic underflow
 #[test]
 fn test_transfer_underflow() {
     ExtBuilder::new(1, 3, 4)
-        .with_initial_balances(vec![(AccountKeyring::Alice.to_account_id(), 499)])
+        .with_initial_balances(vec![(AccountKeyring::Alice.to_account_id(), 400)])
         .build()
         .execute_with(|| {
             run_to_block(1);
+            assert_eq!(Balances::total_issuance(), 400);
             assert_noop!(
                 Balances::transfer_allow_death(
                     frame_system::RawOrigin::Signed(AccountKeyring::Alice.to_account_id()).into(),
                     AccountKeyring::Dave.to_account_id().into(),
-                    500
+                    500 // larger than total issuance
                 ),
                 sp_runtime::ArithmeticError::Underflow
             );
@@ -107,6 +94,7 @@ fn test_transfer_underflow() {
 }
 
 /// test balance transfer without enough balance
+/// can not transfer 500 when only 499 available
 #[test]
 fn test_transfer_funds_unavailable() {
     ExtBuilder::new(1, 3, 4)