test(kernel): replace << with checked_shl in unreachable code that would cause overflow if it was executed

Fixes build failure by `arithmetic_overflow` (deny-by-default).

The Rust compiler reports an `arithmetic_overflow` lint for integer
overflows detected statically. The capability of static analysis is
unspecified (AFAIK), which means the exact set of code that triggers
this lint may vary between compiler versions.

Author: yvt

src/r3_kernel/src/utils/ctz.rs

@@ -314,7 +314,7 @@ mod tests {
                         } else {
                             let low = i & 31;
                             let high = i >> 5;
-                            low | (high << (bits - 5))
+                            low | high.checked_shl((bits - 5) as u32).unwrap()
                         } as usize;
 
                         let got = $func(in_value);