diff --git a/beets/test/helper.py b/beets/test/helper.py
index 124063d76..4effa47f8 100644
--- a/beets/test/helper.py
+++ b/beets/test/helper.py
@@ -48,7 +48,7 @@ from mediafile import Image, MediaFile
 
 import beets
 import beets.plugins
-from beets import autotag, config, importer, logging, util
+from beets import autotag, importer, logging, util
 from beets.autotag.hooks import AlbumInfo, TrackInfo
 from beets.importer import ImportSession
 from beets.library import Album, Item, Library
@@ -153,12 +153,27 @@ def check_reflink_support(path: str) -> bool:
     return reflink.supported_at(path)
 
 
+class ConfigMixin:
+    @cached_property
+    def config(self) -> beets.IncludeLazyConfig:
+        """Base beets configuration for tests."""
+        config = beets.config
+        config.sources = []
+        config.read(user=False, defaults=True)
+
+        config["plugins"] = []
+        config["verbose"] = 1
+        config["ui"]["color"] = False
+        config["threaded"] = False
+        return config
+
+
 NEEDS_REFLINK = unittest.skipUnless(
     check_reflink_support(gettempdir()), "no reflink support for libdir"
 )
 
 
-class TestHelper(_common.Assertions):
+class TestHelper(_common.Assertions, ConfigMixin):
     """Helper mixin for high-level cli and plugin tests.
 
     This mixin provides methods to isolate beets' global state provide
@@ -184,8 +199,6 @@ class TestHelper(_common.Assertions):
         - ``libdir`` Path to a subfolder of ``temp_dir``, containing the
           library's media files. Same as ``config['directory']``.
 
-        - ``config`` The global configuration used by beets.
-
         - ``lib`` Library instance created with the settings from
           ``config``.
 
@@ -202,15 +215,6 @@ class TestHelper(_common.Assertions):
         )
         self.env_patcher.start()
 
-        self.config = beets.config
-        self.config.sources = []
-        self.config.read(user=False, defaults=True)
-
-        self.config["plugins"] = []
-        self.config["verbose"] = 1
-        self.config["ui"]["color"] = False
-        self.config["threaded"] = False
-
         self.libdir = os.path.join(self.temp_dir, b"libdir")
         os.mkdir(syspath(self.libdir))
         self.config["directory"] = os.fsdecode(self.libdir)
@@ -229,8 +233,6 @@ class TestHelper(_common.Assertions):
         self.io.restore()
         self.lib._close()
         self.remove_temp_dir()
-        beets.config.clear()
-        beets.config._materialized = False
 
     # Library fixtures methods
 
@@ -452,7 +454,7 @@ class ItemInDBTestCase(BeetsTestCase):
         self.i = _common.item(self.lib)
 
 
-class PluginMixin:
+class PluginMixin(ConfigMixin):
     plugin: ClassVar[str]
     preload_plugin: ClassVar[bool] = True
 
@@ -473,7 +475,7 @@ class PluginMixin:
         """
         # FIXME this should eventually be handled by a plugin manager
         plugins = (self.plugin,) if hasattr(self, "plugin") else plugins
-        beets.config["plugins"] = plugins
+        self.config["plugins"] = plugins
         beets.plugins.load_plugins(plugins)
         beets.plugins.find_plugins()
 
@@ -494,7 +496,7 @@ class PluginMixin:
         # FIXME this should eventually be handled by a plugin manager
         for plugin_class in beets.plugins._instances:
             plugin_class.listeners = None
-        beets.config["plugins"] = []
+        self.config["plugins"] = []
         beets.plugins._classes = set()
         beets.plugins._instances = {}
         Item._types = getattr(Item, "_original_types", {})
@@ -504,7 +506,7 @@ class PluginMixin:
 
     @contextmanager
     def configure_plugin(self, config: Any):
-        beets.config[self.plugin].set(config)
+        self.config[self.plugin].set(config)
         self.load_plugins(self.plugin)
 
         yield
@@ -624,7 +626,7 @@ class ImportHelper(TestHelper):
     def setup_importer(
         self, import_dir: bytes | None = None, **kwargs
     ) -> ImportSession:
-        config["import"].set_args({**self.default_import_config, **kwargs})
+        self.config["import"].set_args({**self.default_import_config, **kwargs})
         self.importer = self._get_import_session(import_dir or self.import_dir)
         return self.importer
 
diff --git a/test/test_autotag.py b/test/test_autotag.py
index 31afc6a63..7f8ed3d2e 100644
--- a/test/test_autotag.py
+++ b/test/test_autotag.py
@@ -23,7 +23,7 @@ from beets import autotag, config
 from beets.autotag import AlbumInfo, TrackInfo, correct_list_fields, match
 from beets.autotag.hooks import Distance, string_dist
 from beets.library import Item
-from beets.test.helper import BeetsTestCase
+from beets.test.helper import BeetsTestCase, ConfigMixin
 from beets.util import plurality
 
 
@@ -498,11 +498,16 @@ class AlbumDistanceTest(BeetsTestCase):
         assert dist == 0
 
 
-class TestAssignment:
+class TestAssignment(ConfigMixin):
     A = "one"
     B = "two"
     C = "three"
 
+    @pytest.fixture(autouse=True)
+    def _setup_config(self):
+        self.config["match"]["track_length_grace"] = 10
+        self.config["match"]["track_length_max"] = 30
+
     @pytest.mark.parametrize(
         # 'expected' is a tuple of expected (mapping, extra_items, extra_tracks)
         "item_titles, track_titles, expected",