Organize tests - separate unit and integration tests

[vdusek]

• Currently, all our tests are located in the tests/unit/ directory, but not all of them are unit tests.
• Go through all the tests in the tests/unit/ directory and classify them between unit/ and integration/ tests.
• In the future, we should also introduce end-to-end tests, which ideally should execute Crawlers and check their results (using both the MemoryStorageClient and the ApifyStorageClient).

Edit: or we can just have one directory for unit/integration tests and one for e2e.