Adding tests for layout factory

tests/layouts/test_factory.py

@@ -0,0 +1,14 @@
+import pytest
+
+from textual.layouts.dock import DockLayout
+from textual.layouts.factory import get_layout, MissingLayout
+
+
+def test_get_layout_valid_layout():
+    layout_class = get_layout("dock")
+    assert type(layout_class) is DockLayout
+
+
+def test_get_layout_invalid_layout():
+    with pytest.raises(MissingLayout):
+        get_layout("invalid")