### What changes were proposed in this pull request?
Add American timezone during timestamp_seconds doctest
### Why are the changes needed?
`timestamp_seconds` doctest in `functions.py` used default timezone to get expected result
For example:
```python
>>> time_df = spark.createDataFrame([(1230219000,)], ['unix_time'])
>>> time_df.select(timestamp_seconds(time_df.unix_time).alias('ts')).collect()
[Row(ts=datetime.datetime(2008, 12, 25, 7, 30))]
```
But when we have a non-american timezone, the test case will get different test result.
For example, when we set current timezone as `Asia/Shanghai`, the test result will be
```
[Row(ts=datetime.datetime(2008, 12, 25, 23, 30))]
```
So no matter where we run the test case ,we will always get the expected permanent result if we set the timezone on one specific area.
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
Unit test
Closes#28932 from GuoPhilipse/SPARK-32088-fix-timezone-issue.
Lead-authored-by: GuoPhilipse <46367746+GuoPhilipse@users.noreply.github.com>
Co-authored-by: GuoPhilipse <guofei_ok@126.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
ac3a0551d8