Add screenshot test for OSX (kivy#458)

* Add screenshot test for OSX

* Fix style and mkdir errors

* Disable test for Py3 until PyOBJus works

plyer/tests/test_screenshot.py

@@ -0,0 +1,91 @@
+'''
+TestScreenshot
+==============
+
+Tested platforms:
+
+* MacOS
+'''
+
+import unittest
+
+from os import mkdir, remove
+from os.path import join, expanduser, exists
+
+from mock import patch
+from plyer.compat import PY2
+from plyer.tests.common import PlatformTest, platform_import
+
+
+class MockedScreenCapture(object):
+    '''
+    Mocked object used instead of the console-like calling
+    of screencapture binary with parameters.
+    '''
+    @staticmethod
+    def whereis_exe(binary):
+        '''
+        Mock whereis_exe, so that it looks like
+        MacOS screencapture binary is present on the system.
+        '''
+        return binary == 'screencapture'
+
+    @staticmethod
+    def call(args):
+        '''
+        Mocked subprocess.call to check console parameters.
+        '''
+        assert len(args) == 2, len(args)
+        assert args[0] == 'screencapture', args
+        assert args[1] == join(
+            expanduser('~'), 'Pictures', 'screenshot.png'
+        ), args
+        with open(args[1], 'w') as scr:
+            scr.write('')
+
+
+class TestScreenshot(unittest.TestCase):
+    '''
+    TestCase for plyer.screenshot.
+    '''
+
+    def setUp(self):
+        path = join(expanduser('~'), 'Pictures')
+        if not exists(path):
+            mkdir(path)
+
+    @PlatformTest('macosx')
+    def test_screenshot_screencapture(self):
+        '''
+        Test mocked MacOS screencapture for plyer.screenshot.
+        '''
+        if not PY2:
+            print('Can not run until PyOBJus works on Py3.')
+            return
+
+        scr = platform_import(
+            platform='macosx',
+            module_name='screenshot',
+            whereis_exe=MockedScreenCapture.whereis_exe
+        )
+
+        # such class exists in screenshot module
+        self.assertIn('OSXScreenshot', dir(scr))
+
+        # the required instance is created
+        scr = scr.instance()
+        self.assertIn('OSXScreenshot', str(scr))
+
+        # move capture from context manager to run without mock
+        with patch(target='subprocess.call', new=MockedScreenCapture.call):
+            self.assertIsNone(scr.capture())
+
+        scr_path = join(
+            expanduser('~'), 'Pictures', 'screenshot.png'
+        )
+        self.assertTrue(exists(scr_path))
+        remove(scr_path)
+
+
+if __name__ == '__main__':
+    unittest.main()