fix: allow function_casts_as_integer lint in sys-test (#52)

This fixes clippy warnings when running with -D warnings flag.
The warnings were occurring in ctest2-generated code which performs
direct casts of function items to integers. Added the allow attribute
to suppress these warnings in the test code.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: anthropic-code-agent[bot] <242468646+Claude@users.noreply.github.com>
Co-authored-by: Brooooooklyn <3468483+Brooooooklyn@users.noreply.github.com>

Author: Claude

libmimalloc-sys/sys-test/src/main.rs

@@ -1,4 +1,4 @@
-#![allow(bad_style, clippy::all)]
+#![allow(bad_style, clippy::all, function_casts_as_integer)]
 
 use libmimalloc_sys2::*;