[SPARK-23695][PYTHON] Fix the error message for Kinesis streaming tests

## What changes were proposed in this pull request?

This PR proposes to fix the error message for Kinesis in PySpark when its jar is missing but explicitly enabled.

```bash
ENABLE_KINESIS_TESTS=1 SPARK_TESTING=1 bin/pyspark pyspark.streaming.tests
```

Before:

```
Skipped test_flume_stream (enable by setting environment variable ENABLE_FLUME_TESTS=1Skipped test_kafka_stream (enable by setting environment variable ENABLE_KAFKA_0_8_TESTS=1Traceback (most recent call last):
  File "/usr/local/Cellar/python/2.7.14_3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/runpy.py", line 174, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/local/Cellar/python/2.7.14_3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/.../spark/python/pyspark/streaming/tests.py", line 1572, in <module>
    % kinesis_asl_assembly_dir) +
NameError: name 'kinesis_asl_assembly_dir' is not defined
```

After:

```
Skipped test_flume_stream (enable by setting environment variable ENABLE_FLUME_TESTS=1Skipped test_kafka_stream (enable by setting environment variable ENABLE_KAFKA_0_8_TESTS=1Traceback (most recent call last):
  File "/usr/local/Cellar/python/2.7.14_3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/runpy.py", line 174, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/local/Cellar/python/2.7.14_3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/.../spark/python/pyspark/streaming/tests.py", line 1576, in <module>
    "You need to build Spark with 'build/sbt -Pkinesis-asl "
Exception: Failed to find Spark Streaming Kinesis assembly jar in /.../spark/external/kinesis-asl-assembly. You need to build Spark with 'build/sbt -Pkinesis-asl assembly/package streaming-kinesis-asl-assembly/assembly'or 'build/mvn -Pkinesis-asl package' before running this test.
```

## How was this patch tested?

Manually tested.

Author: hyukjinkwon <[email protected]>

Closes apache#20834 from HyukjinKwon/minor-variable.

python/pyspark/streaming/tests.py

@@ -1503,10 +1503,13 @@ def search_flume_assembly_jar():
         return jars[0]
 
 
-def search_kinesis_asl_assembly_jar():
+def _kinesis_asl_assembly_dir():
     SPARK_HOME = os.environ["SPARK_HOME"]
-    kinesis_asl_assembly_dir = os.path.join(SPARK_HOME, "external/kinesis-asl-assembly")
-    jars = search_jar(kinesis_asl_assembly_dir, "spark-streaming-kinesis-asl-assembly")
+    return os.path.join(SPARK_HOME, "external/kinesis-asl-assembly")
+
+
+def search_kinesis_asl_assembly_jar():
+    jars = search_jar(_kinesis_asl_assembly_dir(), "spark-streaming-kinesis-asl-assembly")
     if not jars:
         return None
     elif len(jars) > 1:
@@ -1569,7 +1572,7 @@ def search_kinesis_asl_assembly_jar():
     else:
         raise Exception(
             ("Failed to find Spark Streaming Kinesis assembly jar in %s. "
-             % kinesis_asl_assembly_dir) +
+             % _kinesis_asl_assembly_dir()) +
             "You need to build Spark with 'build/sbt -Pkinesis-asl "
             "assembly/package streaming-kinesis-asl-assembly/assembly'"
             "or 'build/mvn -Pkinesis-asl package' before running this test.")