Adjust software tests for CrateDB

[amotl]

About

Add software tests for GH-5, based on the corresponding vanilla MLflow test cases added with GH-6.

Details

• All tests specific to SQLite and database migrations have been removed.
• With CrateDB's SQLAlchemy polyfill for translating autoincrement column constraints, experiment identifiers are no longer small incrementing integers, but rather wider integers, randomly generated. A few spots need to compensate for that. Specifically, using get_experiment_by_name instead of get_experiment at a few spots made the transition very easy.
• Database tables are now pruned on test setup instead of teardown, in order to make it possible to inspect the database on failed test runs.
• Skip a few test cases still failing.
• Slow tests have been marked with pytest.mark.slow. Running poe check-fast will omit them.

Backlog: Fix more tests

• See Fix or evaluate impact of failing test cases #11.