Move test data download into CMake script

Avoid using shell commands to check if the test directory exists by
using a CMake script.

cmake/json_test.cmake

@@ -12,7 +12,11 @@ else()
     find_package(Git REQUIRED)
     # target to download test data
     add_custom_target(download_test_data
-        COMMAND test -d json_test_data || ${GIT_EXECUTABLE} clone -c advice.detachedHead=false --branch v${JSON_TEST_DATA_VERSION} ${JSON_TEST_DATA_URL} --quiet --depth 1
+        COMMAND ${CMAKE_COMMAND} 
+                    -DGIT_EXECUTABLE=${GIT_EXECUTABLE}
+                    -DJSON_TEST_DATA_VERSION=${JSON_TEST_DATA_VERSION}
+                    -DJSON_TEST_DATA_URL=${JSON_TEST_DATA_URL}
+                    -P ${PROJECT_SOURCE_DIR}/cmake/scripts/clone_test_data.cmake
         COMMENT "Downloading test data from ${JSON_TEST_DATA_URL} (v${JSON_TEST_DATA_VERSION})"
         WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
     )

cmake/scripts/clone_test_data.cmake

@@ -0,0 +1,16 @@
+# clone test data
+
+get_filename_component(test_data_dir json_test_data ABSOLUTE)
+
+if(NOT EXISTS ${test_data_dir})
+    execute_process(COMMAND ${GIT_EXECUTABLE} clone
+                                -q -c advice.detachedHead=false -b v${JSON_TEST_DATA_VERSION} --depth 1
+				                -- ${JSON_TEST_DATA_URL} ${test_data_dir}
+                    RESULT_VARIABLE git_result
+                    OUTPUT_VARIABLE git_output
+                    ERROR_VARIABLE git_output)
+
+    if(NOT git_result EQUAL 0)
+        message(FATAL_ERROR "git failed:\n${git_output}")
+    endif()
+endif()