Try to fix the flaky test force_reindex_commits_from_main_index_spec

What does this MR do and why?

https://gitlab.com/gitlab-org/gitlab/-/jobs/5033526425

I made this spec more robust. Instead of expecting the indexed_documents_count as 0, I am checking the initial_documents_left_to_be_indexed_count to be > 0. And after first batch of .migrate, I am testing the difference between initial_documents_left_to_be_indexed_count - documents_left_to_be_indexed_count to be eq to batch_size

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Related to #419781 (closed)