Resolve "Specs for Gitlab::Database::TablesLocker are very slow"

What does this MR do and why?

Make specs for Gitlab::Database::TablesLocker faster:

• Current master: 2 minutes
• This branch: 28 seconds

The specs for Gitlab::Database::TablesLocker are interacting directly with the database. We lock/unlock all tables during the runtime of the specs. This is very slow.

The Gitlab::Database::TablesLocker is not the responsible class for making the database changes: the database modification is done by Gitlab::Database::LockManager.

So we can mock the LockManager and make these specs run faster.

Related to Issue #390348 (closed)

How to set up and validate locally

Run the specs and check the time it took

MR acceptance checklist

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