Make test caching opt-in with --cached

[eddiebergman]

closes #1447

Use pytest --cached to have autosklearn items cached. If you don't include it, pytest will delete everything in the cache upon invocation.

Items will still be cached between the different tests in the same invocation.

Tested manually:

# Where the cache would exist
$ ls .pytest_cache/d
> . ..

# Slow, has to generate the automl instances
$ pytest test_automl/test_automl/test_performance_over_time.py
$ ls .pytest_cache/d
> . ..  

# Still slow, has to generate the automl instances
$ pytest --cached test_automl/test_automl/test_performance_over_time.py
$ ls .pytest_cache/d
> . ..  autosklearn-cache

# Much faster this time, only has to unpickle automl instances
$ pytest --cached test_automl/test_automl/test_performance_over_time.py
$ ls .pytest_cache/d
> . ..  autosklearn-cache

# Slow again, deletes the cached items this time
$ pytest test_automl/test_automl/test_performance_over_time.py
$ ls .pytest_cache/d
> . ..

[codecov]

Codecov Report

Merging #1464 (6dc3b90) into development (daa9ad6) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           development    #1464      +/-   ##
===============================================
- Coverage        84.31%   84.29%   -0.03%     
===============================================
  Files              147      147              
  Lines            11284    11287       +3     
  Branches          1934     1939       +5     
===============================================
  Hits              9514     9514              
+ Misses            1256     1255       -1     
- Partials           514      518       +4