tests: make test_crash_on_closing_tmpfile_py27 more reliable

[blueyed]

It fails reliable for me now without the fix from 9517c3a, and had
actually a wrong assertion in the end.

Ref: #2370

[codecov]

Codecov Report

Merging #4878 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4878      +/-   ##
==========================================
+ Coverage   94.48%   94.54%   +0.06%     
==========================================
  Files         113      113              
  Lines       25172    25173       +1     
  Branches     2499     2499              
==========================================
+ Hits        23783    23801      +18     
+ Misses       1059     1047      -12     
+ Partials      330      325       -5

Impacted Files	Coverage Δ
testing/test_capture.py	99.24% <100%> (ø)	⬆️
src/_pytest/cacheprovider.py	96.22% <0%> (+0.47%)	⬆️
src/_pytest/terminal.py	91.74% <0%> (+2.64%)	⬆️

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[blueyed]

It was not failing on CI anymore, but looked like it was not covered. added a todo for diff coverage.

But maybe it would be better to just monkeypatch f.close to raise IOError reliably?

[nicoddemus]

But maybe it would be better to just monkeypatch f.close to raise IOError reliably?

Hmm that's an idea. Let's see, if this gets flaky we might go down that route, otherwise I'm fine with the code as it is.

[blueyed]

Ok, squashed.
Let's merge it when it stays green.