Add run_tests_xunit target
Add run_tests_xunit target

Produces xunit compatible XML test statistics report.
Could be useful if running CI independent of BBP infrastructure.

Change-Id: I500cc7ff509f21e11e555125e0f760f5bb6aa7c4

BlueBrain/NeuroM,lidakanari/NeuroM,eleftherioszisis/NeuroM,juanchopanza/NeuroM,liesbethvanherpe/NeuroM,wizmer/NeuroM,mgeplf/NeuroM