Fix: make motion_socket test Python 3.6 compatible

The `capture_output` argument for `subprocess.run` is not available
in Python 3.6, which is the default on Rocky Linux 8.

This commit replaces it with `stdout=subprocess.PIPE` and `stderr=
subprocess.PIPE` to ensure backward compatibility.

This enables `ic-good-opt-*` tests to pass on Rocky 8 environments.

See: #1538

Author: tuhaihe

src/test/regress/expected/motion_socket.out

@@ -48,7 +48,7 @@ for pid in pids_to_check:
     # We check count of those connections which have not been established.
     # Use the regex for example: "TCP <ip>:\d+ .*" (without '->')
     lsof_ret = subprocess.run(["lsof", "-i", "-nP", "-a", "-p", str(pid)],
-        capture_output=True, check=True).stdout
+        stdout=subprocess.PIPE, stderr=subprocess.PIPE, check=True).stdout
     plpy.info(
         f'Checking postgres backend {pid}, ' \
         f'lsof output:\n{os.linesep.join(map(str, lsof_ret.splitlines()))}')

src/test/regress/sql/motion_socket.sql

@@ -50,7 +50,7 @@ for pid in pids_to_check:
     # We check count of those connections which have not been established.
     # Use the regex for example: "TCP <ip>:\d+ .*" (without '->')
     lsof_ret = subprocess.run(["lsof", "-i", "-nP", "-a", "-p", str(pid)],
-        capture_output=True, check=True).stdout
+        stdout=subprocess.PIPE, stderr=subprocess.PIPE, check=True).stdout
     plpy.info(
         f'Checking postgres backend {pid}, ' \
         f'lsof output:\n{os.linesep.join(map(str, lsof_ret.splitlines()))}')