Add integration tests for driver using gRPC

[gavv]

Implement a few simple tests for driver that connect to it via gRPC, add and remove devices, and observe results.

Related files:

• rpc/driver_protocol.proto defines RPC interface
• test - directory for tests (we use GoogleTest)
• tool/connector.cpp - example of client code that connects to driver using gRPC

Example tests:

• call ping(), call driver_info()
• add device (add_device()), check that we can retrieve device info (get_device()) and it has correct parameters, check that it appeared in device list (get_all_devices())
• remove device, check that it was removed from list and info can't be retrieved
• call toggle/bind/connect, and check updated info

These tests will require driver to be loaded, so they should be run after executing make install and make kick. They should be excluded from make test by default and should invoked separately, e.g. using make driver_test.

We should run these tests on CI.