performance-metrics: Reap child process

The snapshot/restore test didn't wait on the child being spawned:

warning: spawned process is never `wait()`ed on
   --> performance-metrics/src/performance_tests.rs:495:25
    |
495 |           let mut child = GuestCommand::new(&guest)
    |  _________________________^
496 | |             .args(["--api-socket", &api_socket_source])
497 | |             .args([
498 | |                 "--cpus",
...   |
507 | |             .spawn()
508 | |             .unwrap();
    | |_____________________^
    |
    = note: consider calling `.wait()`
    = note: not doing so might leave behind zombie processes
    = note: see https://doc.rust-lang.org/stable/std/process/struct.Child.html#warning
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#zombie_processes
    = note: `#[warn(clippy::zombie_processes)]` on by default

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

performance-metrics/src/performance_tests.rs

@@ -518,6 +518,7 @@ pub fn performance_restore_latency(control: &PerformanceTestControl) -> f64 {
         ));
 
         let _ = child.kill();
+        child.wait().unwrap();
 
         let event_path = String::from(guest.tmp_dir.as_path().join("event.json").to_str().unwrap());
         let mut cmd = GuestCommand::new(&guest);