* Save feature info in booster in JSON model.
* [breaking] Remove automatic feature name generation in `DMatrix`.
This PR is to enable reliable feature validation in Python package.
* Do not derive from unittest.TestCase (not needed for pytest)
* assertRaises -> pytest.raises
* Simplify test_empty_dmatrix with test parametrization
* setUpClass -> setup_class, tearDownClass -> teardown_class
* Don't import unittest; import pytest
* Use plain assert
* Use parametrized tests in more places
* Fix test_gpu_with_sklearn.py
* Put back run_empty_dmatrix_reg / run_empty_dmatrix_cls
* Fix test_eta_decay_gpu_hist
* Add parametrized tests for monotone constraints
* Fix test names
* Remove test parametrization
* Revise test_slice to be not flaky
* Disable JSON serialization for now.
* Multi-class classification is checkpointing for each iteration.
This brings significant overhead.
Revert: 90355b4f007ae
* Set R tests to use binary.
* Fix a unit test on CLI, to handle RC versions
* [CI] Use mgpu machine to run gpu hist unit tests
* [CI] Build GPU-enabled JAR artifact and deploy to xgboost-maven-repo
