[SPARK-48714][PYTHON][FOLLOW-UP] Skip tests if test class is not available

[HyukjinKwon]

What changes were proposed in this pull request?

This PR is a followup of #47086 that makes PySpark tests skipped if tests classes are unavailable.

Why are the changes needed?

./build/sbt package should be able to run PySpark tests according to https://spark.apache.org/developer-tools.html and https://spark.apache.org/docs/latest/api/python/development/testing.html

Does this PR introduce any user-facing change?

No, test-only.

How was this patch tested?

Manually tested:

build/sbt -Phive clean package
python/run-tests -k --python-executables python3 --testnames 'pyspark.sql.tests.test_dataframe'

...
Starting test(python3): pyspark.sql.tests.test_dataframe (temp output: /.../spark/python/target/33eca5b9-23e8-4e95-9eca-9f09ce333336/python3__pyspark.sql.tests.test_dataframe__3saz2ymf.log)
Finished test(python3): pyspark.sql.tests.test_dataframe (21s) ... 1 tests were skipped
Tests passed in 21 seconds

Skipped tests in pyspark.sql.tests.test_dataframe with python3:
      test_df_merge_into (pyspark.sql.tests.test_dataframe.DataFrameTests.test_df_merge_into) ... skip (0.001s)

build/sbt -Phive clean test:package
python/run-tests -k --python-executables python3 --testnames 'pyspark.sql.tests.test_dataframe'

Starting test(python3): pyspark.sql.tests.test_dataframe (temp output: /.../spark/python/target/710cf488-f39d-49b4-8b04-70044318ea02/python3__pyspark.sql.tests.test_dataframe___95hp4wt.log)
Finished test(python3): pyspark.sql.tests.test_dataframe (23s)
Tests passed in 23 seconds

Was this patch authored or co-authored using generative AI tooling?

No.

[HyukjinKwon]

Merged to master.

[viirya]

Looks good.