[Task][Playground]: Remove need to manually start Datastore emulator for Playground backend tests

[TSultanov]

What needs to happen?

Playground backend tests should be runnable using only go test command without having to start up Datastore emulator manually. Either
a) remove the need to use emulator entirely by mocking DatastoreServer
b) Automate startup/shutdown of the Datastore emulator from tests code.

Issue Priority

Priority: 3 (nice-to-have improvement)

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