Test BinarySerializer and BinaryParser when XRP value is negative

[sappenin]

In theory, binary-encoded XRP values should never be negative. However, stranger things have occurred and sometimes rippled RPC endpoints return negative values even though they shouldn't. We should augment the BaseSerializerTypeTest and it's associated data-driven-tests.json test-cases to include a few negative XRP values, like -0, -1 and something else (e.g., -100).

Depending on what is decided, we'll likely need to update AmountType#fromJson where it checks for a negative value.