Revert "umu_test: use Popen.wait when waiting for subprocess"

This reverts commit 955ffe9.

umu/umu_run.py

@@ -374,7 +374,6 @@ def run_command(command: list[str]) -> int:
         c_int,
     ] = None
     proc: Popen = None
-    ret: int = 0
     libc: str = get_libc()
 
     if not command:
@@ -407,10 +406,17 @@ def run_command(command: list[str]) -> int:
         start_new_session=True,
         preexec_fn=lambda: prctl(PR_SET_CHILD_SUBREAPER, 1, 0, 0, 0, 0),
     )
-    ret = proc.wait()
-    log.debug("Child %s exited with wait status: %s", proc.pid, ret)
 
-    return ret
+    while True:
+        try:
+            wait_pid, wait_status = os.waitpid(proc.pid, 0)
+            log.debug(
+                "Child %s exited with wait status: %s", wait_pid, wait_status
+            )
+        except ChildProcessError:  # No more children
+            break
+
+    return proc.returncode
 
 
 def main() -> int:  # noqa: D103