📌 use JSON test data v1.0.0

cmake/download_test_data.cmake

 find_package(Git)
 
+set(JSON_TEST_DATA_URL     https://github.com/nlohmann/json_test_data)
+set(JSON_TEST_DATA_VERSION 1.0.0)
+
 # target to download test data
 add_custom_target(download_test_data
-    COMMAND test -d json_test_data || ${GIT_EXECUTABLE} clone https://github.com/nlohmann/json_test_data.git --quiet --depth 1
-    COMMENT "Downloading test data from https://github.com/nlohmann/json_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}.git --quiet --depth 1
+    COMMENT "Downloading test data from ${JSON_TEST_DATA_URL} (v${JSON_TEST_DATA_VERSION})"
     WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
 )