Improve the reporting of integration tests.

[tomprince]

Fixes #482.

• Capture coverage information from all integration tests, instead of just macOS.
• Capture eliot logs from integration tests.
• Turn on windows integration tests. (When I first enabled windows tests in CI, the integration tests seemed to be timing out; that is no longer the case).

I've found myself needing to add these changes to branches to debug things from CI, or re-run the tests locally.

[codecov]

Codecov Report

Merging #483 (b20f73e) into main (b9845af) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #483      +/-   ##
==========================================
+ Coverage   91.46%   91.49%   +0.03%     
==========================================
  Files          35       35              
  Lines        2963     2963              
  Branches      339      339              
==========================================
+ Hits         2710     2711       +1     
+ Misses        181      180       -1     
  Partials       72       72              

Flag	Coverage Δ
integration	70.73% <ø> (+0.06%)	⬆️
unit-test	87.71% <ø> (?)

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Impacted Files	Coverage Δ
src/magic_folder/downloader.py	96.50% <0.00%> (+0.43%)	⬆️

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