python: Move most of the tests to run under a single sandbox

[da-tanabe]

Move most of the tests to run under a single sandbox, but using different Parties.

The remaining tests that have not yet been moved over test more specialized features of dazl, so moving them over to a single sandbox instance is more complicated. Those will be addressed in a subsequent PR.

The TL;DR of the changes:

• Introduce Network.aio_new_party and Network.simple_new_party, which imply an allocation of a new Party. Note that the current implementation does not yet actually call PartyManagementService, though it may in the future. It does, however, guarantee that new Party literals are created (via uuid), which is useful to keep the tests isolated from each other.
• Hardcoded Party literals in tests have been removed.
• Allow ensure_dar to be created before the Network is officially started.
• Introduce an async_network helper method to remove some of the more tedious boilerplate around creating a Network, assigning it a URL, and uploading DARs (and then waiting for those DARs to be fully uploaded).

This is a precursor to switching the tests to run against DAML SDK 1.x (as outlined by #75). As an added benefit, the tests take 7.5 minutes to run instead of 10 minutes (since Sandboxes aren't repeatedly being restarted).