Emit better balance diffs when comparing accounts

[lithp]

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:

• Enable state tests for Constantinople #1181 (comment)
• Failing blockchain tests now emit a state diff #1573 (comment)

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