tests: Factor out a function to create new proxy connections
tests: Factor out a function to create new proxy connections

I'd like to add tests were we have to proxy connections:
  - one to simulate a runtime, another to simulate a shim
  - multiple runtimes/shims

Make it easy to create such new connections.

Signed-off-by: Damien Lespiau <64bd3cb94f359c1a3ce68dae5e26b40578526277@intel.com>

clearcontainers/proxy,dlespiau/clearcontainers-proxy,dlespiau/proxy,clearcontainers/proxy,dlespiau/clearcontainers-proxy,dlespiau/proxy