Fix check_datasets_active and corresponding unit test

[PGijsbers]

This solves an issue hinted at in #149. While retrieving deactivated datasets worked perfectly well, the check_datasets_active method was seemingly not updated for some latest changes, and its unit test was outdated too (it was disabled).

This PR:

• Fixes a bug in check_datasets_active where deactivated datasets would never be found due to not specifying status='all' in the list_datasets call (presumably because the default behavior was changed at some point).
• Correctly points the unit test to the production server, as there is no deactivated dataset on the test server.
• Updated the dataset id that is supposed to refer to an active dataset from 1 to 2 in the unit test (since d/1 is now deactivated).
• reactivates the fixed unit test
• refactored the flow of check_datasets_active a little for legibility and to avoid "excessive" looping/evaluations (minor but still nice :) )

[codecov-io]

Codecov Report

Merging #642 into develop will decrease coverage by 0.21%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #642      +/-   ##
===========================================
- Coverage    90.64%   90.43%   -0.22%     
===========================================
  Files           32       32              
  Lines         3390     3365      -25     
===========================================
- Hits          3073     3043      -30     
- Misses         317      322       +5

Impacted Files	Coverage Δ
openml/datasets/functions.py	96.18% <100%> (+4.13%)	⬆️
openml/datasets/dataset.py	82.4% <0%> (-6.01%)	⬇️
openml/utils.py	91.5% <0%> (-0.56%)	⬇️
openml/runs/functions.py	86.65% <0%> (+0.21%)	⬆️

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[PGijsbers]

Appveyor fails unrelated to this code change, see #643.

[PGijsbers]

Issue with AppVeyor is fixed in #645.