Mock CAVEclient for testing

[ceesem]

Adding a small submodule caveclient.tools.testing that makes it much easier to mock CAVEclients for testing.

1. CAVEclientMock lets you specify which services you anticipate loading, can take a datastack or use a only global-only version, can select what versions of each service you want to test against, specify api version availability, set materialization versions, etc. In general, CAVEclient initialization options are mocked out.

2. CAVEclient conftest.py was altered to use this tooling and several small tests were added to use these features.

3. This caught one or two bugs/issues thanks to these new tests.

[ceesem]

Note on the bugs and issues:

• Global-only CAVEClient initialization was ignoring server_version because it was trying to use the datastack-to-server map when a datastack was not provided. This check was moved to happen only if the condition that a datastack is specified.
• l2cache.has_cache() was always checking the pcg tables anew, rather than doing it once on initialization. Not a huge deal, but it adds latency and also made mocking a pain since it would send a new request through the pcg client with every l2 cache request. Now the pcg-to-l2cache table mapping is saved once retrieved the first time.
• There was no way to tell caveclient to not try to save a synchronized save file for the local server. I realized that this is actually a hazard, because it will save your token to a file automatically and silently without user permission. I changed this to require an explicit call to a new function client.auth.write_local_server_token().

[ceesem]

On reflection, I dont' like the implementation in the third bullet point. Will move that feature to part of the existing save_token function

[ceesem]

Added a parameter local_server to the save_token function which will also save the token to a datastack local server specific file, if a local server is set. Defaults to true, which will effectively save two files with a token for the price of one, making it much more convenient to e.g. use cloudvolume by default to get access to segmentation.