Minimize the use of C Macros for characterization tests

Fewer macros is better!

Introduce a new `JsonChacterizationTest` mixin class to help with this.

Also, avoid some needless copies with `GetParam`.

Part of my effort shoring up the JSON formats with #13570.

src/libutil-test-support/include/nix/util/tests/json-characterization.hh

@@ -0,0 +1,54 @@
+#pragma once
+///@file
+
+#include <gtest/gtest.h>
+#include <nlohmann/json.hpp>
+
+#include "nix/util/types.hh"
+#include "nix/util/file-system.hh"
+
+#include "nix/util/tests/characterization.hh"
+
+namespace nix {
+
+/**
+ * Mixin class for writing characterization tests for `nlohmann::json`
+ * conversions for a given type.
+ */
+template<typename T>
+struct JsonCharacterizationTest : virtual CharacterizationTest
+{
+    /**
+     * Golden test for reading
+     *
+     * @param test hook that takes the contents of the file and does the
+     * actual work
+     */
+    void readJsonTest(PathView testStem, const T & expected)
+    {
+        using namespace nlohmann;
+        readTest(Path{testStem} + ".json", [&](const auto & encodedRaw) {
+            auto encoded = json::parse(encodedRaw);
+            T decoded = adl_serializer<T>::from_json(encoded);
+            ASSERT_EQ(decoded, expected);
+        });
+    }
+
+    /**
+     * Golden test for writing
+     *
+     * @param test hook that produces contents of the file and does the
+     * actual work
+     */
+    void writeJsonTest(PathView testStem, const T & value)
+    {
+        using namespace nlohmann;
+        writeTest(
+            Path{testStem} + ".json",
+            [&]() -> json { return static_cast<json>(value); },
+            [](const auto & file) { return json::parse(readFile(file)); },
+            [](const auto & file, const auto & got) { return writeFile(file, got.dump(2) + "\n"); });
+    }
+};
+
+} // namespace nix

src/libutil-test-support/include/nix/util/tests/meson.build

@@ -7,6 +7,7 @@ headers = files(
   'gmock-matchers.hh',
   'gtest-with-params.hh',
   'hash.hh',
+  'json-characterization.hh',
   'nix_api_util.hh',
   'string_callback.hh',
 )