Added tests for check_md5 and check_integrity (#873)

* fixed check integrity

* stylistic changes

* added test for check_md5 and check_integrity

* flake8

* fix path to test file if not executed from test folder

test/test_datasets_utils.py

 import os
 import shutil
 import tempfile
-import torch
 import torchvision.datasets.utils as utils
 import unittest
 
+TEST_FILE = os.path.join(os.path.dirname(os.path.abspath(__file__)),
+                         'assets', 'grace_hopper_517x606.jpg')
+
 
 class Tester(unittest.TestCase):
 
+    def test_check_md5(self):
+        fpath = TEST_FILE
+        correct_md5 = '9c0bb82894bb3af7f7675ef2b3b6dcdc'
+        false_md5 = ''
+        assert utils.check_md5(fpath, correct_md5)
+        assert not utils.check_md5(fpath, false_md5)
+
+    def test_check_integrity(self):
+        existing_fpath = TEST_FILE
+        nonexisting_fpath = ''
+        correct_md5 = '9c0bb82894bb3af7f7675ef2b3b6dcdc'
+        false_md5 = ''
+        assert utils.check_integrity(existing_fpath, correct_md5)
+        assert not utils.check_integrity(existing_fpath, false_md5)
+        assert utils.check_integrity(existing_fpath)
+        assert not utils.check_integrity(nonexisting_fpath)
+
     def test_download_url(self):
         temp_dir = tempfile.mkdtemp()
         url = "http://github.com/pytorch/vision/archive/master.zip"