fix(test): don't try to kill after CtrlAltDel

In test test_send_ctrl_alt_del we send a CTRL+ALT+DEL to the microVM,
which, in x86, makes the microVM to shutdown. Then we send a signal to
the Firecracker process with `os.kill(firecracker_pid, 0)` and wait for
it to fail. This works but logs an error in the test logs which can be
confusing.

Instead we can call os.waitpid() which waits for the Firecracker process
to exit and returns immediately if the process has already exited.

Signed-off-by: Babis Chalios <bchalios@amazon.es>

tests/integration_tests/functional/test_api.py

@@ -795,19 +795,13 @@ def test_send_ctrl_alt_del(test_microvm_with_api):
 
     # If everything goes as expected, the guest OS will issue a reboot,
     # causing Firecracker to exit.
-    # We'll keep poking Firecracker for at most 30 seconds, waiting for it
-    # to die.
-    start_time = time.time()
-    shutdown_ok = False
-    while time.time() - start_time < 30:
-        try:
-            os.kill(firecracker_pid, 0)
-            time.sleep(0.01)
-        except OSError:
-            shutdown_ok = True
-            break
-
-    assert shutdown_ok
+    # waitpid should block until the Firecracker process has exited. If
+    # it has already exited by the time we call waitpid, WNOHANG causes
+    # waitpid to raise a ChildProcessError exception.
+    try:
+        os.waitpid(firecracker_pid, os.WNOHANG)
+    except ChildProcessError:
+        pass
 
 
 def _drive_patch(test_microvm):