Magma vs Couchstore: dropping all collections doesn't result in 0 items

Whilst debugging MB-42885 I developed some rough changes to the unit tests which dropped all collections during shutdown and validated how many items remained (expecting 0). There are a number of tests which failed for magma for unknown reasons, but pass for couchstore.

The following is a simplified version of the issue which will cause a single failure in the 'HighSeqno' test, the final EXPECT statement fails with 1 item, when 0 is expected.

https://github.com/jimwwalker/kv_engine/commit/744ae368cc1cfa5362347527c05b1d02ae501e48