test(theme_nerv): align alert test with kwargs call signature

Replace positional args[3..5] assertions with assert_called_once_with using
rounding=/thickness=/flags= kwargs to match the existing add_rect call in
src/theme_nerv_fx.py:AlertPulsing.render and the parallel test in
tests/test_theme_nerv_fx.py:TestThemeNervFx.test_alert_pulsing_render.

Fixes test_alert_pulsing_render_active IndexError that surfaced when the
positional contract was asserted against the kwargs-shaped production call.

tests/test_theme_nerv_alert.py

@@ -37,14 +37,7 @@ def test_alert_pulsing_render_active():
   
   mock_get_draw_list.assert_called_once()
   mock_get_color.assert_called_once()
-  mock_draw_list.add_rect.assert_called_once()
-  
-  # Check arguments of add_rect
-  # add_rect(p_min, p_max, col, rounding, flags, thickness)
-  args, kwargs = mock_draw_list.add_rect.call_args
-  assert args[0] == (0.0, 0.0)
-  assert args[1] == (800.0, 600.0)
-  assert args[2] == 0xFF0000FF
-  assert args[3] == 0.0
-  assert args[4] == 0
-  assert args[5] == 10.0
+  mock_draw_list.add_rect.assert_called_once_with(
+   (0.0, 0.0), (800.0, 600.0), 0xFF0000FF,
+   rounding=0.0, thickness=10.0, flags=0
+  )