Don't share objects between TestInstances

[casey]

If two TestInstances are created without providing a list of objects, they'll be shared across both instances.

[laanwj]

Good catch. ACK.
(luckily this case doesn't ever trigger as only one test instance is created per test, but still)

[luke-jr]

Nit: This doesn't actually fix the problem if objects is passed as a parameter. It would be better as:

self.blocks_and_transactions = list(objects)

...or maybe a deep-copy.

But maybe callers would want shared objects in some cases?

Without a real use case, it's hard to know what behaviour is sane here.