[FEATURE] Unit test for swift module

[BECATRUE]

Feature you want to implement

I want to implement the unit test for swift.swift module.

How the feature is implemented

Test for parse()

• TBD

Test for strinfo()

• TBD

Test for Swift class

• TBD

Test for _add_to_path()

• If it doesn't change sys.

Test for _get_argparser()

• If the parser can parse command line arguments.

Test for _read_setup_file()

• If a setup file is opened well.
• If app_infos and bus_infos are assigned using the setup file.

Test for main()

• If each statements is run well.

Additional context

It is necessary to use mock module.

See also: mock

[BECATRUE]

I will fill specific todo-lists later.

[kangz12345]

I think for unit testing, you don't have to write all the plan. The test code itself would tell us explicitly. I guess...?