Fix the tests for disabled implicit conversions and strict NUL handling

- unit-iterators1 (#5835) converted a json element to int implicitly,
  which does not compile with JSON_USE_IMPLICIT_CONVERSIONS=0.
- unit-testsuites parses a number followed by a NUL byte, which
  JSON_STRICT_NUL_HANDLING rejects; leave the NUL out in strict mode.

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

tests/src/unit-iterators1.cpp

@@ -1661,7 +1661,7 @@ TEST_CASE_TEMPLATE("reverse iteration and member access", Json, nlohmann::json,
         seen.clear();
         for (auto r = j.crbegin(); r != j.crend(); ++r)
         {
-            seen.push_back(*r);
+            seen.push_back(r->template get<int>());
         }
         CHECK(seen == std::vector<int> {4, 3, 2, 1});
     }