Fix the tests added by the merged PRs for all CI configurations

- discard the results of dump() and from_*() in CHECK_THROWS with
  utils::ignore_return_value (GCC -Werror=unused-result)
- give unit-bson's huge_string_t a default constructor (MSVC C2512,
  GCC 5, clang 3.5)
- unit-disabled_exceptions: use the literals namespace when the global
  UDLs are off (ci_test_noglobaludls; #5700)
- unit-binary_utf8_strict: expect the JSON pointer prefix with
  JSON_DIAGNOSTICS (#5741)
- skip the tests that rely on exceptions under JSON_NOEXCEPTION
  (#5678, #5732)
- clang-tidy and clang -Werror: static test data, CAPTURE(...);,
  const-correctness, use-after-move alias, unused conversion operator,
  a missing <iterator> include

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

tests/src/unit-allocator.cpp

@@ -567,7 +567,7 @@ struct allocator_no_forward : std::allocator<T>
 {
     allocator_no_forward() = default;
     template <class U>
-    allocator_no_forward(allocator_no_forward<U> /*unused*/) {}
+    allocator_no_forward(const allocator_no_forward<U>& /*unused*/) {}
 
     template <class U>
     struct rebind