Improve widgets tests

[stsewd]

In some places where we need to pass a callback

lira/tests/tui/test_widgets.py

Line 15 in 713fbeb

button = Button("Exit", handler=lambda: print("Hi"))

lira/tests/tui/test_widgets.py

Lines 31 to 33 in 713fbeb

	ListElement("One", on_select=lambda: print("Select")),
	ListElement("Two"),
	ListElement("Three", on_focus=lambda: print("Focus")),

We should actually pass a mock and assert that the mock has been called when expected.