[SPARK-15342] [SQL] [PYSPARK] PySpark test for non ascii column name does not actually test with unicode column name

## What changes were proposed in this pull request?

The PySpark SQL `test_column_name_with_non_ascii` wants to test non-ascii column name. But it doesn't actually test it. We need to construct an unicode explicitly using `unicode` under Python 2.

## How was this patch tested?

Existing tests.

Author: Liang-Chi Hsieh <simonh@tw.ibm.com>

Closes #13134 from viirya/correct-non-ascii-colname-pytest.