Add integration tests for Database trait

[mickvandijke]

Tests run for each database implementation individually. The Sqlite3 test database runs in memory. The Mysql test database runs on a Docker image.

I have included a README.md file in the tests folder with instructions on running tests:

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Running Tests

Torrust requires Docker to run different database systems for testing. install docker here.

Start the databases with docker-compose before running tests:

$ docker-compose up

Run all tests using:

$ cargo test

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Tested functions after this PR:

• insert_user_and_get_id
• get_user_from_id
• get_user_authentication_from_id
• get_user_profile_from_username
• get_user_compact_from_id
• get_user_tracker_key
• count_users
• ban_user
• grant_admin_role
• verify_email
• add_tracker_key
• delete_user
• insert_category_and_get_id
• get_category_from_id
• get_category_from_name
• get_categories
• delete_category
• get_torrents_search_sorted_paginated
• insert_torrent_and_get_id
• get_torrent_from_id
• get_all_torrents_compact
• update_torrent_title
• update_torrent_description
• update_tracker_info
• delete_torrent
• delete_all_database_rows