test: check if destination exists

tests/test_arguments.py

@@ -1,13 +1,29 @@
 from subprocess import CalledProcessError
 import unittest
 import subprocess
+import os
 
 
 class ArgumentTestClass(unittest.TestCase):
 
+  def setUp(self):
+    import tempfile
+    self.existing_dir = tempfile.mkdtemp()
+
   def test_argument_required(self):
     with self.assertRaises(CalledProcessError):
       subprocess.check_output(['python', 'manga_up'])
 
+  def test_destination_exists(self):
+    result = subprocess.check_output(['python', 'manga_up', '--destination', 'nonexistent'])
+    self.assertIn('does not exist', str(result))
+
+  def test_destination_is_not_directory(self):
+    result = subprocess.check_output(['python', 'manga_up', '--destination', self.existing_file])
+    self.assertIn('not a directory', str(result))
+
+  def tearDown(self) -> None:
+      os.rmdir(self.existing_dir)
+
 if __name__ == '__main__':
   unittest.main()