Mark the unordered_set in the hash regression test const

clang-tidy's misc-const-correctness check flagged it: the set is
never mutated after construction, only read via size().

Signed-off-by: Afonso Januário <afonso-januario@hotmail.com>
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-hash.cpp

@@ -145,7 +145,7 @@ TEST_CASE("hash<nlohmann::json>")
     CHECK(std::hash<json> {}(json(-1)) == std::hash<json> {}(json(-1.0)));
 
     // a std::unordered_set relies on this same consistency between == and hash
-    std::unordered_set<json> numbers {json(0), json(static_cast<unsigned>(0)), json(0.0)};
+    const std::unordered_set<json> numbers {json(0), json(static_cast<unsigned>(0)), json(0.0)};
     CHECK(numbers.size() == 1);
 }