Add error message if test suite cannot be found (#3585)

* 🚸 add error message if test suite cannot be found

Fixes #3584

tests/src/make_test_data_available.hpp

@@ -0,0 +1,22 @@
+#pragma once
+
+#include <cstdio>   // fopen, fclose, FILE
+#include <memory> // unique_ptr
+#include <test_data.hpp>
+#include <doctest.h>
+
+namespace utils
+{
+
+inline bool check_testsuite_downloaded()
+{
+    std::unique_ptr<std::FILE, decltype(&std::fclose)> file(std::fopen(TEST_DATA_DIRECTORY "/README.md", "r"), &std::fclose);
+    return file != nullptr;
+}
+
+TEST_CASE("check test suite is downloaded")
+{
+    REQUIRE_MESSAGE(utils::check_testsuite_downloaded(), "Test data not found in '" TEST_DATA_DIRECTORY "'. Please execute target 'download_test_data' before running this test suite. See <https://github.com/nlohmann/json#execute-unit-tests> for more information.");
+}
+
+}  // namespace utils