Emit better balance diffs when comparing accounts

Instead of emitting expected - actual, emit actual - expected.

Rationale (by @veox):

> For example, as things stand, if a fixture-expected value is 1000, and
> the actual produced by py-evm is 500, the printed message will show a
> delta of 500 - that is, a net positive, whereas what truly happened is
> py-evm having produced a value that's too low. This is confusing: say, a
> miner's account balance showing "Delta: 500" makes one think that the
> miner received 500 wei more than expected (which is not true).

Reference:
- ethereum#1181 (comment)
- ethereum#1573 (comment)

eth/tools/fixtures/helpers.py

@@ -78,7 +78,7 @@ def verify_account_db(expected_state: AccountState, account_db: BaseAccountDB) -
                         'balance',
                         actual_value,
                         expected_value,
-                        expected_value - actual_value,
+                        actual_value - expected_value,
                     )
                 )
             else: