Update tests to textual 6.0

Textual added a breaking change where the renderable property
on the Static widget has been changed to content.

Signed-off-by: Pablo Galindo <pablogsal@gmail.com>

Author: pablogsal

tests/unit/test_tui_reporter.py

@@ -141,7 +141,11 @@ async def run_before_wrapper(pilot) -> None:
 
 def render_widget(widget: Widget) -> str:
     output = StringIO()
-    rprint(widget.renderable, file=output)  # type: ignore
+    content = getattr(widget, "renderable", None)
+    if content is None:
+        content = widget.content  # type: ignore
+    assert content is not None
+    rprint(content, file=output)
     return output.getvalue()