Fix CI: missing include and const-correctness

- Add #include <vector> to type_traits.hpp for the new
  is_compatible_binary_type trait's std::vector<std::uint8_t> reference
  (caught by cpplint's include-what-you-use check)
- Mark test-local json variables const where never reassigned
  (caught by clang-tidy's misc-const-correctness check)

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

include/nlohmann/detail/meta/type_traits.hpp

@@ -13,6 +13,7 @@
 #include <tuple> // tuple
 #include <type_traits> // false_type, is_constructible, is_integral, is_same, true_type
 #include <utility> // declval
+#include <vector> // vector
 #if defined(__cpp_lib_byte) && __cpp_lib_byte >= 201603L
     #include <cstddef> // byte
 #endif