Maintenance: Tests: Use explicit encoding for tests

[penguinolog]

• Tests with render can fail if detected encoding differs from used during test write

• add non-public api for internal testing: util.get_encoding() for public getting _target_encoding util.set_temporary_encoding context manager

• Extend "Canvas text is wider than the maxcol specified" details: reproduced on nonstandard encoding

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 7016026786

• 9 of 10 (90.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.08%) to 74.242%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
urwid/canvas.py	0	1	0.0%

Totals
Change from base Build 7015514000:	0.08%
Covered Lines:	7526
Relevant Lines:	10289

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💛 - Coveralls