I wouldn't consider it strange, no. I prefer not to repeat hard-coded strings, so would probably do something like:

    test_string = '12345678'
    app.keyboard.send(test_string[:4])  # 1234
    app.keyboard.tap_backspace()
    app.keyboard.tap_space()
    app.keyboard.send(test_string[4:])  # 5678
    app.keyboard.choose_extended_character('A', 8)  # Æ
    self.assertEqual(app.field, ' '.join([string[:3], string[4:]]) + u'Æ'