Corrected NEKTAR_BUILD_TESTS condition

basics/advection-diffusion/CMakeLists.txt

basics/helmholtz/CMakeLists.txt

events/2011/Cambridge-AIM/CMakeLists.txt

events/2015/Crete-SummerSchool/CMakeLists.txt

events/2015/Imperial-NektarWorkshop/CMakeLists.txt

flow-stability/bfs/CMakeLists.txt

flow-stability/channel/CMakeLists.txt

flow-stability/cylinder/CMakeLists.txt

fundamentals/differentiation/CMakeLists.txt

fundamentals/integration/CMakeLists.txt

incns/taylor-green-vortex/CMakeLists.txt