Test duration of debug_mode tests significantly increasing total time of test suite

[RobinL]

============================== slowest durations ===============================
42.14s call tests/test_debug_mode.py::test_debug_mode_combined_training[spark]
21.49s call tests/test_debug_mode.py::test_debug_mode_ptrrm_train[spark]
20.24s call tests/test_analyse_blocking.py::test_analyse_blocking_slow_methodology[spark]
18.65s call tests/test_debug_mode.py::test_debug_mode_u_training[spark]
17.38s call tests/test_full_example_spark.py::test_full_example_spark
14.38s call tests/test_debug_mode.py::test_debug_mode_em_training[spark]
13.66s call tests/test_debug_mode.py::test_debug_mode_profile_columns[spark]

[RobinL]

One simple solution would be to disable for Spark; i can't remember whether we observed cases where debug mode failed for Spark but nothing else