Split the slowest Valgrind tests out of unit-large_json and unit-bjdata (#5862)

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-bjdata_round_trip_draft2.cpp

@@ -0,0 +1,22 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+using nlohmann::json;
+
+#include "bjdata_round_trip.hpp"
+
+// split from unit-bjdata.cpp, see bjdata_round_trip.hpp; the other half is in
+// unit-bjdata_round_trip_draft3.cpp
+
+TEST_CASE("BJData round-trip invariants (j1 written with Draft 2)")
+{
+    utils::check_bjdata_round_trip(json::bjdata_version_t::draft2);
+}