Do not parse the value in the array-at() test

JSON_DIAGNOSTIC_POSITIONS adds the byte range of the value to the exception
message, which a parsed value has and an in-memory one does not, so the two
message checks failed in that configuration. Build the array in memory
instead of parsing it; the test is about at(size_type) not needing
array_t::at(), and the byte range is beside the point.

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

tests/src/unit-custom-array-type.cpp

@@ -105,7 +105,9 @@ TEST_CASE("array type without capacity()")
 
 TEST_CASE("array type without at()")
 {
-    auto j = no_at_json::parse(R"([1,2,3])");
+    // built in memory rather than parsed, so that the exception message does
+    // not gain a byte range with JSON_DIAGNOSTIC_POSITIONS
+    no_at_json j = {1, 2, 3};
     const auto& jc = j;
 
     CHECK(j.at(0) == 1);