[SPARK-34886][PYTHON] Port/integrate Koalas DataFrame unit test into …

…PySpark

### What changes were proposed in this pull request?
Now that we merged the Koalas main code into the PySpark code base (#32036), we should port the Koalas DataFrame unit test to PySpark.

### Why are the changes needed?
Currently, the pandas-on-Spark modules are not tested at all. We should enable the DataFrame unit test first.

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
Enable the DataFrame unit test.

Closes #32083 from xinrong-databricks/port.test_dataframe.

Authored-by: Xinrong Meng <xinrong.meng@databricks.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

dev/sparktestsupport/modules.py

@@ -610,6 +610,8 @@ def __hash__(self):
         "pyspark.pandas.spark.accessors",
         "pyspark.pandas.spark.utils",
         "pyspark.pandas.typedef.typehints",
+        # unittests
+        "pyspark.pandas.tests.test_dataframe",
     ],
     excluded_python_implementations=[
         "PyPy"  # Skip these tests under PyPy since they require numpy, pandas, and pyarrow and

python/pyspark/pandas/testing/__init__.py

@@ -0,0 +1,16 @@
+#
+# Licensed to the Apache Software Foundation (ASF) under one or more
+# contributor license agreements.  See the NOTICE file distributed with
+# this work for additional information regarding copyright ownership.
+# The ASF licenses this file to You under the Apache License, Version 2.0
+# (the "License"); you may not use this file except in compliance with
+# the License.  You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
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+#