Adding a warning message when HudsonTestCase and JenkinsRule reach the...

* Adding a warning message when HudsonTestCase and JenkinsRule reach the configured timeout. This message will be very useful to diagnose test failures caused by timeouts.

* Adding a mechanism to allow users running tests in slow systems can change the default timeout. Now they can change this value using the environment variable "jenkins.test.timeout" expresed in seconds. Example: mvn test -Djenkins.test.timeout=300