refactor(test): add more debugging for "jailer id still used"

Clean up the error messages we print when running into the "firecracker
pid was killed, but process with its jailer id still exists" case: Also
print the PID of the offending process, as well as filter out
non-firecracker processes at the command-running stage (otherwise the
error message always contained the grep process itself, which was
confusing).

Note that `cmd` must be the last column, as otherwise ps will truncate
it.

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

tests/framework/microvm.py

@@ -333,12 +333,12 @@ def kill(self):
             # https://github.com/firecracker-microvm/firecracker/pull/4442/commits/d63eb7a65ffaaae0409d15ed55d99ecbd29bc572
 
             # filter ps results for the jailer's unique id
-            _, stdout, stderr = utils.check_output(
-                f"ps ax -o cmd -ww | grep {self.jailer.jailer_id}"
+            _, stdout, stderr = utils.run_cmd(
+                f"ps ax -o pid,cmd -ww | grep {self.jailer.jailer_id} | grep firecracker"
             )
             # make sure firecracker was killed
             assert (
-                stderr == "" and "firecracker" not in stdout
+                not stderr and not stdout
             ), f"Firecracker reported its pid {self.firecracker_pid}, which was killed, but there still exist processes using the supposedly dead Firecracker's jailer_id: {stdout}"
 
         if self.uffd_handler and self.uffd_handler.is_running():