Add tests.

[fukatani]

I investigated code which has especially low coverage.
(from https://coveralls.io/builds/25971809 )

• login.py: but service is not found
• split_input: almost all part is not tested.
• test_reactive: almost all part is not tested.
• utils.exec_command: FileNotFoundError and PermissionError
• download_history: not tested.
• test.py: compare_as_floats -> #538: Add float compare test. #540
• test.py: compare_mode==line -> #438: Add test for line mode. #541
• test.py: tle -> Issue/438/rstrip and tle #543
• test.py: rstrip -> Issue/438/rstrip and tle #543
• dispatch.py -> #538: Add get_latest_version_test #548

These are also not covered, but it seems difficult to test

• login_with_browser
• FormSender
• DummySubmission
• hackerrank.py and other contests: _get_lang_display_mapping and get_available_languages

I know this report is not perfect, your suggentions are welcome.

[fukatani]

I recommend make one PR for one untested item.