Make tests independent from running Vasum server instance

Make tests independent from running Vasum server instance

[Bug]           Running tests when Vasum server was up caused "Connection
                refused" error in vasum-cli.
[Cause]         Unit tests used vasum-ipc.socket, which overriden the
                connection between CLI and Vasum server.
[Solution]      Separate unit-tests IPC socket from Vasum server socket.
                Unit-tests now create and use their own socket:
                vasum-ipc-unit-tests.socket
[Verification]  Build, install, run Vasum service, run all tests.
                Ensure that CLI works after tests finish running.

Change-Id: Ic94500b743e990273361d85dffecfbe17ddc0347