Remove nose-related tests

I am a maintainer of Python packages in openSUSE, and I am on my crusade of eliminating nose1 from our distribution. When I look at its repository on https://github.com/nose-devs/nose, the last release 1.3.7 was on 2 Jun 2015, and even the last commit on the master branch was on 4 Mar 2016. Moreover it will stop working with Python 3.10.

[This patch](https://github.com/box/flaky/files/5679645/remove_nose.patch.txt) eliminates dependency on nose, which has been unnecessary for a long time.

Start by reviewing the linked remove_nose.patch.txt and the repository's test setup to identify every nose-related change. The work is done when the nose dependency and related test usage are removed without losing coverage, and the existing test suite passes using the supported runner.

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