Complete coverage of arrangement doc options

While the code in _arrange.py does mark the unknown dock type branch as
being for the benefit of type checkers only, and was marked as to be ignored
for the purposes of coverage, it was still showing in a coverage test and
it's not a terrible thing to ensure that a user, giving the wrong type of
dock in their own code, will get the expected result.

tests/test_arrange.py

@@ -1,3 +1,5 @@
+import pytest
+
 from textual._arrange import arrange, TOP_Z
 from textual._layout import WidgetPlacement
 from textual.geometry import Region, Size, Spacing
@@ -91,3 +93,9 @@ def test_arrange_dock_bottom():
     ]
     assert widgets == {child, header}
     assert spacing == Spacing(0, 0, 1, 0)
+
+def test_arrange_dock_badly():
+    child = Widget(id="child")
+    child.styles.dock = "nowhere"
+    with pytest.raises(AssertionError):
+        _ = arrange( Widget(), [child], Size(80, 24), Size(80, 24))