As #69219 outlines the unit tests in `csi_client_test.go` where not
testing the actual implementation of the `csiDriverClient` but was
testing the fake.

To fix this, we changed the `csiDriverClient` to use a
`nodeClientCreator` which is responsible for creating a new
`NodeClient`, a real one in prod and a fake one in the tests.

The setup of the gRPC connection has been pushed into that creator. The
node client uses that connection; that's transparent to the driver
client. It's the responsibility of the driver client to close the
connection when it is done with the node client. To achieve this, we
have the node client creator return a closer which handles the
connection teardown.

In the tests we now also check if the driver client actually calls
this closer, thus closing the gRPC connection.

Closes: #69219
Co-authored-by: Rosie Bloxsom <rbloxsom@pivotal.io>
Co-authored-by: Maria Ntalla <mntalla@pivotal.io>