Add integration tests

[alexashley]

scenarios:

• single index, handful of documents: after the migration, the source index should be gone and all documents should exist in the new index
• induce an artificial failure after each step, the migration should still complete successfully
• multiple migrations begin running at the same time
• multiple migrations with a staggered start

To make triggering the migrations easier, we could use fstest.MapFS to point the index manager at an in-memory filesystem (this is already done in the tests for MappingsRegistry).