This was decided at the latest release manager meeting:
To get rid of the top level tests/ directory, the UnitTests.xml
file is moved to the core extension to a new folder Build/. This
directory will later be used for other build specific things.

Change-Id: Id541f1e0bab3b1d2891f13697f48ea16e6c86d5e
Resolves: #40837
Releases: 6.0
Reviewed-on: http://review.typo3.org/14528
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn