Unittests: refactor creation of fake datasets and embeddings

[MalteEbner]

Currently the unittests create fake datasets and embeddings at several different locations and in serveral different ways. This could be reworked to have only one function for each.

Locations:

• https://github.com/lightly-ai/lightly/blob/36e1bf05c78f928b7a75b7e253e51c00e5c79240/tests/api_workflow/test_api_workflow_upload_dataset.py#L25
• https://github.com/lightly-ai/lightly/blob/36e1bf05c78f928b7a75b7e253e51c00e5c79240/tests/api_workflow/test_api_workflow_upload_dataset.py#L45
• https://github.com/lightly-ai/lightly/blob/36e1bf05c78f928b7a75b7e253e51c00e5c79240/tests/api_workflow/test_api_workflow_upload_embeddings.py#L41
• https://github.com/lightly-ai/lightly/blob/36e1bf05c78f928b7a75b7e253e51c00e5c79240/tests/cli/test_cli_magic.py#L34
• https://github.com/lightly-ai/lightly/blob/36e1bf05c78f928b7a75b7e253e51c00e5c79240/tests/cli/test_cli_embed.py#L30
• https://github.com/lightly-ai/lightly/blob/36e1bf05c78f928b7a75b7e253e51c00e5c79240/tests/cli/test_cli_crop.py#L37
• https://github.com/lightly-ai/lightly/blob/36e1bf05c78f928b7a75b7e253e51c00e5c79240/tests/cli/test_cli_train.py#L29
• https://github.com/lightly-ai/lightly/blob/36e1bf05c78f928b7a75b7e253e51c00e5c79240/tests/cli/test_cli_upload.py#L25