misc/mxnet: don't build the tests if testing is not enabled

Local (testport) builds were failing like so:

ld: error: undefined symbol: testing::Message::GetString() const
>>> referenced by base.cc
>>>               tests/CMakeFiles/mxnet_unit_tests.dir/cpp/misc/base.cc.o:(testing::AssertionResult::AppendMessage(testing::Message const&))

because the testing libraries were not being linked -- because
they're not even searched for when testing is not enabled.
Extend previous patch that put the searching-for-testlibs
behind a check, to apply the same check to building-tests.

misc/mxnet/files/patch-CMakeLists.txt

@@ -1,6 +1,6 @@
---- CMakeLists.txt.orig	2019-12-13 19:55:11 UTC
+--- CMakeLists.txt.orig	2020-07-15 07:03:28 UTC
 +++ CMakeLists.txt
-@@ -490,6 +490,7 @@ if(USE_JEMALLOC)
+@@ -464,6 +464,7 @@ if(USE_JEMALLOC)
    endif()
  endif()
  
@@ -8,15 +8,15 @@
  include(CTest)
  set(GTEST_ROOT "${CMAKE_CURRENT_SOURCE_DIR}/3rdparty/googletest/googletest")
  set(GTEST_INCLUDE_DIR ${GTEST_ROOT}/include)
-@@ -500,6 +501,7 @@ set(GTEST_LIBRARY gtest)
+@@ -474,6 +475,7 @@ set(GTEST_LIBRARY gtest)
  
  add_subdirectory(${GTEST_ROOT})
  find_package(GTest REQUIRED)
 +endif()
  
  # cudnn detection
- if(USE_CUDNN AND USE_CUDA)
-@@ -819,7 +821,7 @@ install(TARGETS ${MXNET_INSTALL_TARGETS}
+ if(USE_CUDNN)
+@@ -865,7 +867,7 @@ install(TARGETS ${MXNET_INSTALL_TARGETS}
  #       https://cmake.org/cmake/help/v3.0/module/GNUInstallDirs.html
  
  install(DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/3rdparty/dlpack/include/ DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
@@ -25,3 +25,13 @@
  if(USE_MKLDNN)
    install(DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/3rdparty/mkldnn/include/ DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
  endif()
+@@ -913,7 +915,9 @@ endif()
+ if(BUILD_CPP_EXAMPLES)
+   add_subdirectory(example/image-classification/predict-cpp)
+ endif()
++if(ENABLE_TESTING)
+ add_subdirectory(tests)
++endif()
+ 
+ # ---[ Linter target
+ if(MSVC)