Fix test lints that are red on develop (effc++, clang-tidy) (#5946)

- unit-bson: initialize the SAX recorder's member (-Weffc++)
- unit-json_patch: build the reference document independently instead of
  copying it (performance-unnecessary-copy-initialization)
- unit-json_view_builder: use node_kind::link instead of an out-of-range
  enum cast (clang-analyzer-optin.core.EnumCastOutOfRange)

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

tests/src/unit-bson.cpp

@@ -1311,7 +1311,7 @@ TEST_CASE("BSON declared document size bounds the content")
     // records the events that a SAX parser receives
     struct recorder : nlohmann::json_sax<json>
     {
-        std::string events;
+        std::string events {};
 
         bool null() override
         {