Try to fix #745 by providing ModuleSpec for __main__.

[tjwalton]

I investigated #745 and found the main difference (compared to running the unit test without coverage) seemed to be the lack of a __spec__ for sys.modules['__main__'], so I tried to add one.

I also added a test which triggers the problem based off @adamklein's example in the issue.

However, after the fix we don't get coverage inside the spawned process, and it locks up MultiprocessingTest when using spawn. I don't know why though. Suggestions would be appreciated.

[codecov-io]

Codecov Report

Merging #836 into master will decrease coverage by 8.98%.
The diff coverage is 95.45%.

@@            Coverage Diff             @@
##           master     #836      +/-   ##
==========================================
- Coverage   92.34%   83.35%   -8.99%     
==========================================
  Files          83       83              
  Lines       11121    11243     +122     
  Branches     1133     1146      +13     
==========================================
- Hits        10270     9372     -898     
- Misses        715     1712     +997     
- Partials      136      159      +23

Impacted Files	Coverage Δ
coverage/backward.py	98.38% <100%> (+0.05%)	⬆️
tests/helpers.py	100% <100%> (ø)	⬆️
coverage/execfile.py	82.95% <100%> (+0.19%)	⬆️
tests/test_concurrency.py	92.59% <94.44%> (-5.48%)	⬇️
tests/plugin2.py	0% <0%> (-62.97%)	⬇️
tests/test_plugins.py	44.78% <0%> (-54.46%)	⬇️
tests/test_context.py	60.46% <0%> (-39.54%)	⬇️
tests/test_api.py	60.11% <0%> (-36.12%)	⬇️
tests/test_oddball.py	66.93% <0%> (-33.07%)	⬇️
tests/test_debug.py	65.97% <0%> (-31.96%)	⬇️
... and 25 more

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[nedbat]

Thanks for starting this. I fixed it another way.