Use Boost.Operators in more places

Change-Id: I7024c3e7d4c30c99ec7a895d484f227cb60fb356
diff --git a/tests/unit/metadata-object.t.cpp b/tests/unit/metadata-object.t.cpp
index 480f7d4..0cbe737 100644
--- a/tests/unit/metadata-object.t.cpp
+++ b/tests/unit/metadata-object.t.cpp
@@ -26,6 +26,9 @@
 
 namespace ndn::tests {
 
+static_assert(std::is_convertible_v<MetadataObject::Error*, tlv::Error*>,
+              "MetadataObject::Error must inherit from tlv::Error");
+
 class MetadataObjectFixture : public KeyChainFixture
 {
 public: