Only run tests if tests are enabled

[awesomebytes]

On a system where tests are not run by default ros_numpy fails to build with:

CMake Error at /home/nao/gentoo/opt/ros/kinetic/share/catkin/cmake/test/tests.cmake:18 (message):
  () is not available when tests are not enabled.  The CMake code should only
  use it inside a conditional block which checks that testing is enabled:

  if(CATKIN_ENABLE_TESTING)

    (...)

  endif()

Call Stack (most recent call first):
  CMakeLists.txt:181 (catkin_add_nosetests)

So... just following with what is suggested.

[awesomebytes]

Just a little bump @eric-wieser , and reminding I offered myself in the PR: #6 as a maintainer of the package to release new versions and so.

[eric-wieser]

It looks like you've found a bug upstream in ROS here too - there's supposed to be something before those ()