Avoid TestHelper dependency in MediaFile tests

Part of #1966. This also introduces a nicer, more lightweight way to create and
destroy temporary directories, decoupling that functionality from the
giant morass that is TestHelper.

test/_common.py

@@ -333,6 +333,28 @@ class Bag(object):
         return self.fields.get(key)
 
 
+# Convenience methods for setting up a temporary sandbox directory for tests
+# that need to interact with the filesystem.
+
+class TempDirMixin(object):
+    """Text mixin for creating and deleting a temporary directory.
+    """
+
+    def create_temp_dir(self):
+        """Create a temporary directory and assign it into `self.temp_dir`.
+        Call `remove_temp_dir` later to delete it.
+        """
+        path = tempfile.mkdtemp()
+        if not isinstance(path, bytes):
+            path = path.encode('utf8')
+        self.temp_dir = path
+
+    def remove_temp_dir(self):
+        """Delete the temporary directory created by `create_temp_dir`.
+        """
+        shutil.rmtree(self.temp_dir)
+
+
 # Platform mocking.
 
 @contextmanager

test/test_mediafile.py

@@ -20,7 +20,6 @@ from __future__ import division, absolute_import, print_function
 
 import os
 import shutil
-import tempfile
 import datetime
 import time
 import unittest
@@ -293,7 +292,8 @@ class GenreListTestMixin(object):
         assertCountEqual(self, mediafile.genres, [u'the genre', u'another'])
 
 
-class ReadWriteTestBase(ArtTestMixin, GenreListTestMixin):
+class ReadWriteTestBase(ArtTestMixin, GenreListTestMixin,
+                        _common.TempDirMixin):
     """Test writing and reading tags. Subclasses must set ``extension`` and
     ``audio_properties``.
     """
@@ -378,14 +378,10 @@ class ReadWriteTestBase(ArtTestMixin, GenreListTestMixin):
     ]
 
     def setUp(self):
-        name = tempfile.mkdtemp()
-        if not isinstance(name, bytes):
-            name = name.encode('utf8')
-        self.temp_dir = name
+        self.create_temp_dir()
 
     def tearDown(self):
-        if os.path.isdir(self.temp_dir):
-            shutil.rmtree(self.temp_dir)
+        self.remove_temp_dir()
 
     def test_read_nonexisting(self):
         mediafile = self._mediafile_fixture('full')

test/test_mediafile_edge.py

@@ -23,7 +23,6 @@ import unittest
 import mutagen.id3
 
 from test import _common
-from test.helper import TestHelper
 
 from beets import mediafile
 import six
@@ -145,7 +144,7 @@ class InvalidValueToleranceTest(unittest.TestCase):
         self.assertEqual(v, 1.0)
 
 
-class SafetyTest(unittest.TestCase, TestHelper):
+class SafetyTest(unittest.TestCase, _common.TempDirMixin):
     def setUp(self):
         self.create_temp_dir()
 
@@ -210,7 +209,7 @@ class SideEffectsTest(unittest.TestCase):
         self.assertEqual(old_mtime, new_mtime)
 
 
-class MP4EncodingTest(unittest.TestCase, TestHelper):
+class MP4EncodingTest(unittest.TestCase, _common.TempDirMixin):
     def setUp(self):
         self.create_temp_dir()
         src = os.path.join(_common.RSRC, b'full.m4a')
@@ -229,7 +228,7 @@ class MP4EncodingTest(unittest.TestCase, TestHelper):
         self.assertEqual(new_mf.label, u'foo\xe8bar')
 
 
-class MP3EncodingTest(unittest.TestCase, TestHelper):
+class MP3EncodingTest(unittest.TestCase, _common.TempDirMixin):
     def setUp(self):
         self.create_temp_dir()
         src = os.path.join(_common.RSRC, b'full.mp3')
@@ -332,7 +331,7 @@ class SoundCheckTest(unittest.TestCase):
         self.assertEqual(peak, 0.0)
 
 
-class ID3v23Test(unittest.TestCase, TestHelper):
+class ID3v23Test(unittest.TestCase, _common.TempDirMixin):
     def _make_test(self, ext=b'mp3', id3v23=False):
         self.create_temp_dir()
         src = os.path.join(_common.RSRC,