Replace big export test data files with on the fly generated data

[sphuber]

Fixes #1632

Two export and import tests were relying on rather large export archives
to test the import functionality. However, since these archives were
included in the main aiida package, they would also be included in the
distribution, causing the package to become unnecessarily heavy. By
replacing these archives and tests with a variant that generates the
test data archive on the fly, the package size will be reduced by almost
have, while maintaining the same test coverage.

[codecov-io]

Codecov Report

Merging #1645 into release_v0.12.1 will decrease coverage by 0.04%.
The diff coverage is n/a.

@@                 Coverage Diff                 @@
##           release_v0.12.1    #1645      +/-   ##
===================================================
- Coverage            54.71%   54.66%   -0.05%     
===================================================
  Files                  246      246              
  Lines                32420    32420              
===================================================
- Hits                 17739    17723      -16     
- Misses               14681    14697      +16

Impacted Files	Coverage Δ
aiida/common/pluginloader.py	65.64% <0%> (-10.69%)	⬇️
aiida/orm/importexport.py	79.05% <0%> (-0.21%)	⬇️

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[ltalirz]

while maintaining the same test coverage.

I'm seeing Coverage 54.71% 54.66% -0.05%

;-) just kidding - this is great, thanks a lot!

[sphuber]

Since Codecov hasn't proven itself to be right most of the time, I am going to chalk this one up to Codecov :)