[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 #25315 from WeichenXu123/fix_py37_daemon.

Authored-by: WeichenXu <[email protected]>
Signed-off-by: HyukjinKwon <[email protected]>

Author: WeichenXu123

python/pyspark/tests/test_daemon.py

@@ -47,6 +47,9 @@ 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)