[SPARK-50406][PYTHON][TESTS] Improve pyspark.sql.tests.test_udtf

### What changes were proposed in this pull request?
Improve pyspark.sql.tests.test_udtf by
- extract `udtf_for_table_argument` for code reuse
- use assertDataFrameEqual universally

### Why are the changes needed?
Code cleanup.

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

### How was this patch tested?
Test changes only.

### Was this patch authored or co-authored using generative AI tooling?
No.

Closes #48949 from xinrong-meng/impr_udtf_test.

Authored-by: Xinrong Meng <[email protected]>
Signed-off-by: Xinrong Meng <[email protected]>