Revert "Remove RNN tests"

tmva/tmva/test/DNN/RNN/CMakeLists.txt

tmva/tmva/test/DNN/RNN/TestForwardPass.cxx

tmva/tmva/test/DNN/RNN/TestForwardPass.h

tmva/tmva/test/DNN/RNN/TestForwardPassCpu.cxx

tmva/tmva/test/DNN/RNN/TestForwardPassCuda.cxx

tmva/tmva/test/DNN/RNN/TestFullRNN.cxx

tmva/tmva/test/DNN/RNN/TestFullRNN.h

tmva/tmva/test/DNN/RNN/TestFullRNNCpu.cxx

tmva/tmva/test/DNN/RNN/TestFullRNNCuda.cxx

tmva/tmva/test/DNN/RNN/TestRecurrentBackpropagation.cxx

tmva/tmva/test/DNN/RNN/TestRecurrentBackpropagation.h

tmva/tmva/test/DNN/RNN/TestRecurrentBackpropagationCpu.cxx

tmva/tmva/test/DNN/RNN/TestRecurrentBackpropagationCuda.cxx