Reduce the time of tests

[wdevazelhes]

Tests take a significant time to run, especially tests that need to run on datasets that have a reasonable size. Possible options to make the tests go faster are:

• Put the max_iter (if applicable) of estimators to 1 in the metric_learners listing from test_utils.py
• Reduce the size of datasets in test_utils.py as much as possible while still passing the tests
  ...