Mark the code of json_view that tests cannot reach

The 4 GiB limit and the fallback for an input that parse() accepts but
the view rejects (a bug) are excluded from the coverage; shrink_to_fit()
of an empty document is tested.

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

include/nlohmann/json_view.hpp

@@ -459,7 +459,7 @@ class basic_json_document
         bool ok = false;
         if (NLOHMANN_VIEW_UNLIKELY(size >= 0xFFFFFFF0u))
         {
-            failure.code = detail::view::error_code::input_too_large;
+            failure.code = detail::view::error_code::input_too_large; // LCOV_EXCL_LINE (4 GiB)
         }
         else
         {