Refactor handling of test mojos

maven-failsafe-plugin deserves the same level of integration
as maven-surefire-plugin.

A failed IT should mark the build as unstable, not failed.
[JENKINS-4951] distinguish-test-failures-and-errors
Merge branch 'distinguish-test-failures-and-errors' of https://github.com/tbrugz/jenkins into distinguish-test-failures-and-errors