[SPARK-23847][FOLLOWUP][PYTHON][SQL] Actually test [desc|acs]_nulls_[first|last] functions in PySpark

## What changes were proposed in this pull request?

There was a mistake in `tests.py` missing `assertEquals`.

## How was this patch tested?

Fixed tests.

Author: hyukjinkwon <[email protected]>

Closes apache#21035 from HyukjinKwon/SPARK-23847.

Author: HyukjinKwon

python/pyspark/sql/tests.py

@@ -2991,19 +2991,23 @@ def test_create_dateframe_from_pandas_with_dst(self):
                 os.environ['TZ'] = orig_env_tz
             time.tzset()
 
-    def test_2_4_functions(self):
+    def test_sort_with_nulls_order(self):
         from pyspark.sql import functions
 
         df = self.spark.createDataFrame(
             [('Tom', 80), (None, 60), ('Alice', 50)], ["name", "height"])
-        df.select(df.name).orderBy(functions.asc_nulls_first('name')).collect()
-        [Row(name=None), Row(name=u'Alice'), Row(name=u'Tom')]
-        df.select(df.name).orderBy(functions.asc_nulls_last('name')).collect()
-        [Row(name=u'Alice'), Row(name=u'Tom'), Row(name=None)]
-        df.select(df.name).orderBy(functions.desc_nulls_first('name')).collect()
-        [Row(name=None), Row(name=u'Tom'), Row(name=u'Alice')]
-        df.select(df.name).orderBy(functions.desc_nulls_last('name')).collect()
-        [Row(name=u'Tom'), Row(name=u'Alice'), Row(name=None)]
+        self.assertEquals(
+            df.select(df.name).orderBy(functions.asc_nulls_first('name')).collect(),
+            [Row(name=None), Row(name=u'Alice'), Row(name=u'Tom')])
+        self.assertEquals(
+            df.select(df.name).orderBy(functions.asc_nulls_last('name')).collect(),
+            [Row(name=u'Alice'), Row(name=u'Tom'), Row(name=None)])
+        self.assertEquals(
+            df.select(df.name).orderBy(functions.desc_nulls_first('name')).collect(),
+            [Row(name=None), Row(name=u'Tom'), Row(name=u'Alice')])
+        self.assertEquals(
+            df.select(df.name).orderBy(functions.desc_nulls_last('name')).collect(),
+            [Row(name=u'Tom'), Row(name=u'Alice'), Row(name=None)])
 
 
 class HiveSparkSubmitTests(SparkSubmitTests):