Update get_available_demos tests

[fealho]

Version: SDV 1.0.0 (in progress)

Problem Description

Both test_get_available_demos_single_table and test_get_available_demos_multi_table depend on the exact datasets present in S3. Whenever new datasets are added, these tests crash.

Solution

Update both of these tests so they only validate that a few datasets were correctly fetched, instead of all of them.