Link rendering tests to OSMesa library, not include dir

There was an error in the CMakeLists.txt in the rendering tests where
the include directory was linked in instead of the OSMesa library.
It was a simple mistake of typing the wrong CMake variable.

vtkm/rendering/testing/CMakeLists.txt

@@ -28,7 +28,7 @@ if (OSMESA_FOUND)
   set(unit_tests ${unit_tests}
     UnitTestMapperOSMesa.cxx
     )
-  list(APPEND libs ${OSMESA_INCLUDE_DIR})
+  list(APPEND libs ${OSMESA_LIBRARY})
 endif()
 
 VTKM_unit_tests(SOURCES ${unit_tests}