test(kernel): use "alternate" hexadecimal formatting

The "alternate" flag (`#`) prepends the output with a hexadecimal
literal prefix `0x`.

Author: yvt

src/r3_kernel/src/utils/int.rs

@@ -280,7 +280,7 @@ mod tests {
 
                     let got_set_bits: Vec<u32> = i.one_digits().collect();
 
-                    log::trace!("i = 0x{i:x}");
+                    log::trace!("i = {i:#x}");
                     log::trace!("    got = {got_set_bits:?}");
                     log::trace!("    expected = {set_bits:?}");