[SPARK-23132][PYTHON][ML] Run doctests in ml.image when testing

## What changes were proposed in this pull request?

This PR proposes to actually run the doctests in `ml/image.py`.

## How was this patch tested?

doctests in `python/pyspark/ml/image.py`.

Author: hyukjinkwon <[email protected]>

Closes #20294 from HyukjinKwon/trigger-image.

(cherry picked from commit 45ad97d)
Signed-off-by: hyukjinkwon <[email protected]>

Author: HyukjinKwon

python/pyspark/ml/image.py

@@ -194,9 +194,9 @@ def readImages(self, path, recursive=False, numPartitions=-1,
         :return: a :class:`DataFrame` with a single column of "images",
                see ImageSchema for details.
 
-        >>> df = ImageSchema.readImages('python/test_support/image/kittens', recursive=True)
+        >>> df = ImageSchema.readImages('data/mllib/images/kittens', recursive=True)
         >>> df.count()
-        4
+        5
 
         .. versionadded:: 2.3.0
         """
@@ -216,3 +216,25 @@ def readImages(self, path, recursive=False, numPartitions=-1,
 def _disallow_instance(_):
     raise RuntimeError("Creating instance of _ImageSchema class is disallowed.")
 _ImageSchema.__init__ = _disallow_instance
+
+
+def _test():
+    import doctest
+    import pyspark.ml.image
+    globs = pyspark.ml.image.__dict__.copy()
+    spark = SparkSession.builder\
+        .master("local[2]")\
+        .appName("ml.image tests")\
+        .getOrCreate()
+    globs['spark'] = spark
+
+    (failure_count, test_count) = doctest.testmod(
+        pyspark.ml.image, globs=globs,
+        optionflags=doctest.ELLIPSIS | doctest.NORMALIZE_WHITESPACE)
+    spark.stop()
+    if failure_count:
+        exit(-1)
+
+
+if __name__ == "__main__":
+    _test()