Fix MinLoop.watch_pipe regression for the callback outcome not False

[penguinolog]

Start adding unittests for MainLoop since it had ~0% of real coverage

Fix: #847

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

[coveralls]

Pull Request Test Coverage Report for Build 8078284597

Details

• -1 of 5 (80.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.9%) to 74.364%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
urwid/event_loop/main_loop.py	4	5	80.0%

Totals
Change from base Build 8069409807:	1.9%
Covered Lines:	9274
Relevant Lines:	12611

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💛 - Coveralls