Avoid implicit unsigned char to char conversion in stream tests

Building a std::string from the iterators of a CBOR byte vector narrows
each byte, which -fsanitize=implicit-conversion reports for bytes above
0x7F. Construct the string from the raw bytes instead.

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

tests/src/unit-stream-bulk-scan.cpp

@@ -371,7 +371,7 @@ TEST_CASE("parsing from a std::istream scans the get area in bulk")
     {
         const json j = {{"string", std::string(300, 'x')}, {"array", {1, 2, 3}}, {"number", 1.5}};
         const std::vector<std::uint8_t> cbor = json::to_cbor(j);
-        const std::string bytes(cbor.begin(), cbor.end());
+        const std::string bytes(reinterpret_cast<const char*>(cbor.data()), cbor.size());
         for (const std::size_t chunk_size : chunk_sizes())
         {
             CAPTURE(chunk_size)