test: fix tests when '-' is absent from kernel version (#6681)

On my machine, any test that calls `is_pidfd_available` fails as my
kernel string does not contain a '-'; the code expects there to be one.

Fix the test so that is works regardless on whether the kernel string
contains a '-'.

tokio/src/process/unix/pidfd_reaper.rs

@@ -245,7 +245,12 @@ mod test {
         assert!(status.success());
         let stdout = String::from_utf8_lossy(&stdout);
 
-        let mut kernel_version_iter = stdout.split_once('-').unwrap().0.split('.');
+        let mut kernel_version_iter = match stdout.split_once('-') {
+            Some((version, _)) => version,
+            _ => &stdout,
+        }
+        .split('.');
+
         let major: u32 = kernel_version_iter.next().unwrap().parse().unwrap();
         let minor: u32 = kernel_version_iter.next().unwrap().parse().unwrap();