Fix path to cmake file.

Fix path to cmake file.

The current path leads to a failure on my system when calling
'ninja setup_tests':

FAILED: cd /node/bangerth/trunk/build/tests/sharedtria && /w/bangerth/share/software/cmake-2.8.12.2/bin/cmake -GNinja -DDEAL_II_DIR=/node/bangerth/trunk/build -UDIFF_DIR -UNUMDIFF_DIR -UTEST_PICKUP_REGEX -UTEST_TIME_LIMIT /node/bangerth/trunk/dealii/tests/sharedtria > /dev/null
CMake Error at CMakeLists.txt:2 (INCLUDE):
  include could not find load file:

    ../tests/setup_testsubproject.cmake

CMake Error at CMakeLists.txt:4 (INCLUDE):
  include called with wrong number of arguments.  Include only takes one
  file.

Fix this by providing the same path as in the tests/distributed
directory.