Fix flaky tests involving scripttest's result.files_updated

[cjerdonek]

Looks like it's failed a few times on test_force_reinstall_with_same_version_specifier(), and only on the Python 2.7 job.

This is what the intermittent failure looks like:

assert result2.files_updated, 'force-reinstall failed'
AssertionError: force-reinstall failed
assert {}
 +  where {} = <tests.lib.TestPipResult object at 0x10913cb90>.files_updated

My guess is that this is an issue with the scripttest library when running on Mac, specifically with how equality is computed with the files_updated dict:

https://github.com/pypa/scripttest/blob/01c7db96ff01de034bd7781a4ed28d74eb716d32/scripttest.py#L391-L394

You can even see that someone filed a similar issue four years ago in pip's tracker: #2580

Someone should probably file an issue in scripttest's tracker.

Originally posted by @cjerdonek in #6043 (comment)

[pradyunsg]

@cjerdonek Since you'd raised concerns about scripttest during our conversation at PyCon, I just realized that there's something kinda-sorta similar at: https://github.com/kvas-it/pytest-console-scripts

[ichard26]

Have we faced flaky failures involving scripttest's result.files_updated lately? If not, let's close this.

[uranusjr]

I don’t recall this being a problem recently.

[ichard26]

Let's close this then.