Fix clang-tidy performance-inefficient-string-concatenation in unit-json_view_builder

Fixes the ci_clang_tidy failure at tests/src/unit-json_view_builder.cpp:549.

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

tests/src/unit-json_view_builder.cpp

@@ -546,7 +546,11 @@ TEST_CASE("json_view builder: fraction digits")
             const std::string number = std::string(rng() % 2 == 0 ? "-" : "") + std::to_string(rng() % 100000) + "." + fraction;
             check_same(number);
             check_same("[" + number + "]");
-            check_same("[" + number + "," + number + "]");
+            std::string pair = "[" + number;
+            pair += ",";
+            pair += number;
+            pair += "]";
+            check_same(pair);
             check_same("{\"a\":" + number + "}");
             for (const char follower : followers)
             {