json_view tests: use unsigned literals for the accept() nesting depths

ci_static_analysis_clang reports -Wsign-conversion for the int literals that
initialize 'const std::size_t depth' in the range-for over nesting depths.

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

tests/src/unit-json_view_builder.cpp

@@ -476,7 +476,7 @@ TEST_CASE("json_view builder")
         // nesting beyond the 64 levels that are kept on the builder's own stack, closed and not closed
         for (const std::size_t depth :
                 {
-                    63, 64, 65, 66, 200, 1000
+                    63u, 64u, 65u, 66u, 200u, 1000u
                 })
         {
             check_accept(std::string(depth, '[') + std::string(depth, ']'));