### What changes were proposed in this pull request?
Improve the error message in test GroupedMapInPandasTests.test_grouped_over_window_with_key to show the incorrect values.
### Why are the changes needed?
This test failure has come up often in Arrow testing because it tests a struct with timestamp values through a Pandas UDF. The current error message is not helpful as it doesn't show the incorrect values, only that it failed. This change will instead raise an assertion error with the incorrect values on a failure.
Before:
```
======================================================================
FAIL: test_grouped_over_window_with_key (pyspark.sql.tests.test_pandas_grouped_map.GroupedMapInPandasTests)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/spark/python/pyspark/sql/tests/test_pandas_grouped_map.py", line 588, in test_grouped_over_window_with_key
self.assertTrue(all([r[0] for r in result]))
AssertionError: False is not true
```
After:
```
======================================================================
ERROR: test_grouped_over_window_with_key (pyspark.sql.tests.test_pandas_grouped_map.GroupedMapInPandasTests)
----------------------------------------------------------------------
...
AssertionError: {'start': datetime.datetime(2018, 3, 20, 0, 0), 'end': datetime.datetime(2018, 3, 25, 0, 0)}, != {'start': datetime.datetime(2020, 3, 20, 0, 0), 'end': datetime.datetime(2020, 3, 25, 0, 0)}
```
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
Improved existing test
Closes#28987 from BryanCutler/pandas-grouped-map-test-output-SPARK-32162.
Authored-by: Bryan Cutler <cutlerb@gmail.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
1d1809636b