Release the test value with = nullptr instead of reset()

clang-tidy (readability-ambiguous-smartptr-reset-call) rejects j.reset()
because the pointee inherits a reset() from counting_base.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N5NasAgbDGG28MqXjQvLqp
Signed-off-by: Claude <noreply@anthropic.com>

tests/src/unit-custom-base-class.cpp

@@ -544,7 +544,7 @@ TEST_CASE("destroying a value does not construct the custom base class")
     const auto check_destroy = [](std::unique_ptr<json_with_counting_base> j, std::size_t values)
     {
         counting_base::reset();
-        j.reset();
+        j = nullptr;
         CHECK(counting_base::constructions == 0);
         CHECK(counting_base::destructions == values);
     };