Rename "test" folder to "tests"

All other repositories have tests in “tests” folder. For more
information see:

http://lists.wikimedia.org/pipermail/qa/2014-July/001663.html
http://lists.wikimedia.org/pipermail/qa/2014-July/001667.html
http://lists.wikimedia.org/pipermail/qa/2014-July/001672.html

Change-Id: Ib1e4fa4b7203bd8217615db1ceac23a329c081f1
Merge "Rename "test" folder to "tests""