[#727] Run PrefixNetworkScenario in clean environment each time

Description

Problem: test_PrefixNetworkScenario fails if it's run in the same tezos-client environment more than once with TwoMoneybagKeys exception.

Solution: Since we're mangling the network env anyway, we can just create a new tmpdir for test_PrefixNetworkScenario to run in each time. We do need to remove the directory after the test has run.

Related issue(s)

Resolves #727 (closed)

That said, the issue description wasn't updated after a little discussion.

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