[MINOR][PYTHON][TESTS] Reduce the python worker error log of `test_to…

…DF_with_schema_string`

### What changes were proposed in this pull request?
Reduce the python worker error log of `test_toDF_with_schema_string`

### Why are the changes needed?
When I run the test locally
```python
python/run-tests -k --python-executables python3 --testnames 'pyspark.sql.tests.test_dataframe'
```

Two assertions in `test_toDF_with_schema_string` generate too many python worker error logs (~1k lines), which easily exceed the limitation of terminal and make it hard to debug.

So I want to reduce the number of python workers in the two assertions.

### Does this PR introduce _any_ user-facing change?
no, test only

### How was this patch tested?
manually test, the logs will be reduced to ~200 lines

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

Closes #48388 from zhengruifeng/test_to_df_error.

Authored-by: Ruifeng Zheng <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

python/pyspark/sql/tests/test_dataframe.py

@@ -506,14 +506,16 @@ def test_toDF_with_schema_string(self):
 
         # number of fields must match.
         self.assertRaisesRegex(
-            Exception, "FIELD_STRUCT_LENGTH_MISMATCH", lambda: rdd.toDF("key: int").collect()
+            Exception,
+            "FIELD_STRUCT_LENGTH_MISMATCH",
+            lambda: rdd.coalesce(1).toDF("key: int").collect(),
         )
 
         # field types mismatch will cause exception at runtime.
         self.assertRaisesRegex(
             Exception,
             "FIELD_DATA_TYPE_UNACCEPTABLE",
-            lambda: rdd.toDF("key: float, value: string").collect(),
+            lambda: rdd.coalesce(1).toDF("key: float, value: string").collect(),
         )
 
         # flat schema values will be wrapped into row.