Write test fixtures as code instead of crafting by-hand

[ca-johnson]

The workflow for modifying test server fixture is to run unit tests, pause them halfway, make changes and save the resulting fixture. (See test_perforce.py:test_fixture)

Instead, it would be better to write as code:

server = setup_server()
server.add_stream("my-stream")

client = setup_client(server, stream="my-stream")
changelist = client.new_changelist()
changelist.add_file('file.txt', content="Hello World")
client.submit(changelist)

changelist = client.new_changelist()
changelist.add_file('file.txt', content="Goodbye World")
client.shelve(changelist)

This would setup a server with one stream, where a client submitted a changelist and holds a shelved changelist.

This way, we can more easily create and test different scenarios.