Add tests for the initialisation of CSS_PATH

This improves coverage of _make_path_object_relative in _path.py, and
generally provides some simple tests of how CSS_PATH gets turned into a
css_path property of an App.

Note that if an actual CSS path gets added (something that has been mooted)
this will need updating.

tests/test_path.py

@@ -0,0 +1,43 @@
+from typing import Type
+from pathlib import Path
+
+from textual.app import App
+
+class RelativePathObjectApp(App[None]):
+
+    CSS_PATH = Path("test.css")
+
+class RelativePathStrApp(App[None]):
+
+    CSS_PATH = "test.css"
+
+class AbsolutePathObjectApp(App[None]):
+
+    CSS_PATH = Path("/tmp/test.css")
+
+class AbsolutePathStrApp(App[None]):
+
+    CSS_PATH = "/tmp/test.css"
+
+def path_tester(obj_type: Type[App[None]], str_type: Type[App[None]], intended_result: Path) -> None:
+    assert isinstance(obj_type().css_path,Path), (
+        "CSS_PATH didn't stay as an object"
+    )
+    assert isinstance(str_type().css_path,Path), (
+        "CSS_PATH wasn't converted from str to Path"
+    )
+    assert obj_type().css_path == intended_result, (
+        "CSS_PATH doesn't match the intended result."
+    )
+    assert str_type().css_path == intended_result, (
+        "CSS_PATH doesn't match the intended result."
+    )
+    assert str_type().css_path == obj_type().css_path, (
+        "CSS_PATH str to Path conversion gave a different result"
+    )
+
+def test_relative_path():
+    path_tester(RelativePathObjectApp, RelativePathStrApp, ((Path(__file__).absolute().parent ) / "test.css"))
+
+def test_absolute_path():
+    path_tester(AbsolutePathObjectApp, AbsolutePathStrApp, Path("/tmp/test.css"))