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

## What changes were proposed in this pull request?

This PR picks up https://github.com/apache/spark/pull/25315 back after removing `Popen.wait` usage which exists in Python 3 only. I saw the last test results wrongly and thought it was passed.

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

## How was this patch tested?

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 #25343 from HyukjinKwon/SPARK-28582.

Authored-by: WeichenXu <weichen.xu@databricks.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

python/pyspark/tests/test_daemon.py

@@ -47,6 +47,9 @@ class DaemonTests(unittest.TestCase):
         # 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)