[test/test_compat] Restore the old value of the HOME environment variable

If the test was run before the YoutubeIE tests (for example by running
"nosetests -v test/test_compat.py test/test_download.py -m 'Youtube_1|compat_expand'"),
it wrote the signatures cache to the 'C:\Documents and Settings\тест\Application Data' folder.
It failed due to a problem in the cache code and the write_json_file function (fixed in f03e33b89a)

test/test_compat.py

@@ -26,11 +26,13 @@ class TestCompat(unittest.TestCase):
         self.assertEqual(compat_getenv('YOUTUBE-DL-TEST'), test_str)
 
     def test_compat_expanduser(self):
+        old_home = os.environ.get('HOME')
         test_str = 'C:\Documents and Settings\тест\Application Data'
         os.environ['HOME'] = (
             test_str if sys.version_info >= (3, 0)
             else test_str.encode(get_filesystem_encoding()))
         self.assertEqual(compat_expanduser('~'), test_str)
+        os.environ['HOME'] = old_home
 
     def test_all_present(self):
         import youtube_dl.compat