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-element_access2.cpp

@@ -1963,8 +1963,8 @@ TEST_CASE("operator[] with user-defined std::string_view-convertible types")
     };
 
     json j = {{"foo", "from_class"}, {"bar", "from_struct"}};
-    TestClass foo_obj;
-    TestStruct bar_obj;
+    const TestClass foo_obj;
+    const TestStruct bar_obj;
 
     SECTION("read access")
     {
@@ -2005,6 +2005,10 @@ TEST_CASE("keys convertible to std::string_view work with all lookup functions (
     // 3.12.0, such a key worked with at, the const operator[], find, count and
     // contains via the conversion to std::string; #4958 made the KeyType&&
     // templates win overload resolution for it instead, and those then failed
+    // the lookups pick the conversion to std::string_view, which leaves the one
+    // to std::string unused; it has to exist to reproduce the ambiguity
+    DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
+    DOCTEST_CLANG_SUPPRESS_WARNING("-Wunused-member-function")
     struct DualKey
     {
         operator std::string() const
@@ -2016,6 +2020,7 @@ TEST_CASE("keys convertible to std::string_view work with all lookup functions (
             return "a";
         }
     };
+    DOCTEST_CLANG_SUPPRESS_WARNING_POP
 
     SECTION("nlohmann::json")
     {