Fix clang-tidy errors in json_view tests on develop (#5989)

readability-math-missing-parentheses in unit-json_view_builder.cpp (#5974)
and performance-inefficient-string-concatenation in
unit-json_view_positions.cpp.

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

tests/src/unit-json_view_builder.cpp

@@ -692,7 +692,7 @@ TEST_CASE("json_view builder: fraction digits")
             check_same(number);
             check_same("[" + number + "]");
             std::string pair;
-            pair.reserve(number.size() * 2 + 3);
+            pair.reserve((number.size() * 2) + 3);
             pair += '[';
             pair += number;
             pair += ',';