[SPARK-28582][PYSPARK] Fix flaky test DaemonTests.do_termination_test which fail on Python 3.7

Fix flaky test DaemonTests.do_termination_test which fail on Python 3.7. I add a sleep after the test connection to daemon.

Run test
```
python/run-tests --python-executables=python3.7 --testname "pyspark.tests.test_daemon DaemonTests"
```
**Before**
Fail on test "test_termination_sigterm". And we can see daemon process do not exit.
**After**
Test passed

Closes apache#25315 from WeichenXu123/fix_py37_daemon.

Authored-by: WeichenXu <[email protected]>
Signed-off-by: HyukjinKwon <[email protected]>
(cherry picked from commit fbeee0c)
Signed-off-by: HyukjinKwon <[email protected]>

Author: WeichenXu123

python/pyspark/tests.py

@@ -1916,7 +1916,7 @@ def test_malformed_RDD(self):
 
 class DaemonTests(unittest.TestCase):
     def connect(self, port):
-        from socket import socket, AF_INET, SOCK_STREAM
+        from socket import socket, AF_INET, SOCK_STREAM# request shutdown
         sock = socket(AF_INET, SOCK_STREAM)
         sock.connect(('127.0.0.1', port))
         # send a split index of -1 to shutdown the worker
@@ -1939,9 +1939,12 @@ def do_termination_test(self, terminator):
         # daemon should accept connections
         self.assertTrue(self.connect(port))
 
+        # wait worker process spawned from daemon exit.
+        time.sleep(1)
+
         # request shutdown
         terminator(daemon)
-        time.sleep(1)
+        daemon.wait(5)
 
         # daemon should no longer accept connections
         try: