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

## What changes were proposed in this pull request?

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 #25315 from WeichenXu123/fix_py37_daemon.

Authored-by: WeichenXu <b702da6e50975f71fc6618ef763b578cd392d17f@databricks.com>
Signed-off-by: HyukjinKwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>

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