Add post-migration test table

What does this MR do and why?

Add post-migration test table

As part of gitlab-com/gl-infra&585 (closed), we need to test the execution of post-migrations in the post-deploy pipeline. Given post-migrations are not common, we decided to test with a fake post-migration, this commit introduces one that creates a fake table, the table is going to be removed after the testing is done.

Related to gitlab-com/gl-infra/delivery#2352 (closed)

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