Some recent changes, and updates to third-party tools, have regressed

some unit tests. This change fixes them up.
Fixes include:

A cookie-related error assertion that didn't work on all versions of

  Python 2 and 3.
Full-repository unit tests never cleaned up their temp directories.
Modern versions of flake8 no longer report both styling errors and

  syntax errors in our test file.
RuboCop had a bad string check.
RuboCop error message checks now check in a way that doesn't depend on

  a given version of Ruby.
Shellcheck error checks have been updated for string changes in the

  tool.

All unit tests pass on Python 2.7 and 3.6-3.10 (except Infer, which I

can't yet test on Apple M1).