TestShell: Return self from setup()

This allows user to chain setup() to the initializer. test-shell.md code
examples have been updated to reflect this.

test/functional/test_framework/test_shell.py

@@ -42,6 +42,7 @@ class TestShell:
 
             super().setup()
             self.running = True
+            return self
 
         def shutdown(self):
             if not self.running: