Try to improve theme tests (#6848)

* Try to make DesignTest a fixture

* Try by copying modifiers

* Try to clear state.stylesheets

panel/tests/theme/test_base.py

@@ -1,13 +1,21 @@
 import pathlib
 
+import pytest
+
 from bokeh.models import ImportedStyleSheet
 
 from panel.io.resources import CDN_DIST
+from panel.io.state import state
 from panel.theme.base import BOKEH_DARK, Design, Inherit
 from panel.viewable import Viewable
 from panel.widgets import FloatSlider, IntSlider, TextInput
 
 
+@pytest.fixture(autouse=True)
+def _clear_state():
+    state._stylesheets.clear()
+
+
 class DesignTest(Design):
 
     modifiers = {