Fix ConfigTest failures locally

A couple of `ConfigTest` would previously fail locally since they
somehow depended on the local environment to work fine. This commmit
decouples them from any environment by setting up patching of the
environment variables properly.

test/test_ui.py

@@ -831,12 +831,9 @@ def setUp(self):
         # Don't use the BEETSDIR from `helper`. Instead, we point the home
         # directory there. Some tests will set `BEETSDIR` themselves.
         del os.environ["BEETSDIR"]
-        self._old_home = os.environ.get("HOME")
-        os.environ["HOME"] = os.fsdecode(self.temp_dir)
 
         # Also set APPDATA, the Windows equivalent of setting $HOME.
-        self._old_appdata = os.environ.get("APPDATA")
-        os.environ["APPDATA"] = os.fsdecode(
+        appdata_dir = os.fsdecode(
             os.path.join(self.temp_dir, b"AppData", b"Roaming")
         )
 
@@ -846,9 +843,7 @@ def setUp(self):
 
         # Default user configuration
         if platform.system() == "Windows":
-            self.user_config_dir = os.path.join(
-                self.temp_dir, b"AppData", b"Roaming", b"beets"
-            )
+            self.user_config_dir = os.path.join(appdata_dir, b"beets")
         else:
             self.user_config_dir = os.path.join(
                 self.temp_dir, b".config", b"beets"
@@ -861,19 +856,19 @@ def setUp(self):
         # Custom BEETSDIR
         self.beetsdir = os.path.join(self.temp_dir, b"beetsdir")
         os.makedirs(syspath(self.beetsdir))
+        self.env_patcher = patch(
+            "os.environ",
+            {"HOME": os.fsdecode(self.temp_dir), "APPDATA": appdata_dir},
+        )
+        self.env_patcher.start()
 
         self._reset_config()
         self.load_plugins()
 
     def tearDown(self):
+        self.env_patcher.stop()
         commands.default_commands.pop()
         os.chdir(syspath(self._orig_cwd))
-        if self._old_home is not None:
-            os.environ["HOME"] = self._old_home
-        if self._old_appdata is None:
-            del os.environ["APPDATA"]
-        else:
-            os.environ["APPDATA"] = self._old_appdata
         self.unload_plugins()
         super().tearDown()