[Enhancement] Add test coverage reports

[kdmukai]

Add test suite coverage reports

Note: The current test suite only runs on Raspi hardware (#339 will move testing to non-Raspi local dev).

Changes

• Adds new test-only dependency in tests/requirements.txt for coverage.
• .coveragerc configuration.
• See the usage notes for coverage in the tests/README.md.
• Additional tests in test_mnemonic_generation.py to prototype improving test coverage.

tldr

pip install coverage
coverage run -m pytest
coverage report
coverage html

Open the resulting coverage html. Click into any listed file and explore which lines of code the tests did or did not execute.

[jdlcdl]

ACK tested.

This worked for me on the pi2.
same: as of be7d8be

I left some notes on code lines above, where my eye caught something obvious. already corrected in b05a16d

I really like this pr as it gives us a way to quantify our test coverage and better yet to go hunt down where we can improve it. I'm all for merging this as soon as reviewed ... so that it's absence isn't a blocker for Keith's other branches/prs, and so other can build on this too.

Nicely done!

[newtonick]

ACK, tested

This is a great PR. I'm familiar with the concept of code coverage but never used "coverage" in python.

I was able to run and see the coverage report on cli and review the html output version after copying over from the pi to my computer. I skimmed through the config without understanding most of it. I don't think this requires my understanding and should not hold up merging this PR.