build warning on Mac

'sprintf' has been explicitly marked deprecated on Mac when build

```
In file included from yalantinglibs/src/util/tests/main.cpp:18:
yalantinglibs/src/include/doctest.h:3735:1: warning: 'sprintf' is deprecated: This function is provided for compatibility reasons only. Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead. [-Wdeprecated-declarations]
3735 | DOCTEST_TO_STRING_OVERLOAD(int long long unsigned, "%llu")
| ^
yalantinglibs/src/include/doctest.h:3721:14: note: expanded from macro 'DOCTEST_TO_STRING_OVERLOAD'
3721 | std::sprintf(buf, fmt, in); \
| ^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.2.sdk/usr/include/_stdio.h:274:1: note: 'sprintf' has been explicitly marked deprecated here
274 | __deprecated_msg("This function is provided for compatibility reasons only. Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead.")
| ^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.2.sdk/usr/include/sys/cdefs.h:218:48: note: expanded from macro '__deprecated_msg'
218 | #define __deprecated_msg(_msg) __attribute__((__deprecated__(_msg)))
```

Reproduce the Mac build for yalantinglibs/src/util/tests/main.cpp and inspect yalantinglibs/src/include/doctest.h around lines 3721-3735, where the warning points to sprintf. Done means the test build no longer emits this Mac deprecation warning.

Written by the indexing model from the issue text.