No test capture

Allow execution of tests in multiple forked VM. Within a VM tests are executed sequentially, making possible output capture while having multiple tests run concurrently. This works well together with testHideSuccessfulOutput.
To activate this feature, forking must be activated.
The number of forked VM is set by testNumberForkedJvm. When set to more than one, intra-vm parallel execution is disabled.

This is being discussed in

    https://groups.google.com/forum/#!topic/sbt-dev/E7gPr8iu9b4
    https://groups.google.com/d/msg/sbt-dev/w7uli_-gQe8/FyhFYPhLX7sJ