Make unit tests faster

[dumbledad]

At present running all the unit tests in WSL on my laptop takes 42 minutes. That is too long for me to run all the tests locally before pushing any changes I have made. The unit tests would be more useful if they ran so fast that I would routinely run them all locally while I work on a PR.

The tests that run for more than a minute (in WSL on my XPS15) are:

394.35s call Tests/AfterTraining/test_after_training.py::test_submit_for_inference[True]
207.80s call Tests/SSL/test_ssl_containers.py::test_innereye_ssl_container_cifar10_resnet_simclr
127.20s call Tests/SSL/test_data_modules.py::test_innereye_vision_module
123.78s call Tests/ML/utils/test_checkpoint_handling.py::test_get_best_checkpoint_single_run
62.47s call Tests/ML/utils/test_checkpoint_handling.py::test_get_recovery_path_train_single_run
62.26s call Tests/ML/utils/test_checkpoint_handling.py::test_download_checkpoints_from_single_run
62.14s call Tests/ML/utils/test_checkpoint_handling.py::test_get_checkpoints_to_test_single_run
61.99s call Tests/ML/test_download_upload.py::test_download_recovery_single_run

AB#4166

[dumbledad]

Note that 11 fail locally which pass in our Azure build pipeline:

FAILED Tests/AfterTraining/test_after_training.py::test_submit_for_inference[False] - azureml.exceptions._azureml_exc...
FAILED Tests/AfterTraining/test_after_training.py::test_submit_for_inference[True] - azureml.exceptions._azureml_exce...
FAILED Tests/AfterTraining/test_after_training.py::test_recovery_on_2_nodes - azureml.exceptions._azureml_exception.S...
FAILED Tests/ML/test_download_upload.py::test_download_azureml_dataset - NotImplementedError: Linux distribution ubun...
FAILED Tests/ML/test_lightning_containers.py::test_create_fastmri_container - ModuleNotFoundError: No module named 'f...
FAILED Tests/ML/test_lightning_containers.py::test_run_fastmri_container - ValueError: Model name FastMriOnRandomData...
FAILED Tests/ML/visualizers/test_plot_cross_validation.py::test_add_comparison_data - Exception: Run refs_pull_444_me...
FAILED Tests/ML/visualizers/test_visualize_patches.py::test_visualize_patch_sampling[True] - AssertionError: Image si...
FAILED Tests/ML/visualizers/test_visualize_patches.py::test_visualize_patch_sampling_2d - AssertionError: Image sizes...
FAILED Tests/SSL/test_data_modules.py::test_innereye_vision_module - NameError: name 'cv2' is not defined
FAILED Tests/SSL/test_ssl_containers.py::test_innereye_ssl_container_cifar10_resnet_simclr - NameError: name 'cv2' is...