texture: Clear the existing texture cache before testing

This test started failing after updating to pytest 8.0. I fix it by
clearing the cache so the test can begin with a clear slate.

Signed-off-by: Anna Schumaker <Anna@NoWheyCreamery.com>

tests/test_texture.py

@@ -12,6 +12,10 @@ from gi.repository import Gdk
 class TestTextureCache(unittest.TestCase):
     """Test our custom cache dictionary."""
 
+    def setUpClass():
+        """Clear the existing cache before testing."""
+        emmental.texture.CACHE.clear()
+
     def setUp(self):
         """Set up common variables."""
         cover = tests.util.COVER_JPG.absolute().relative_to("/")