CBG-3838 fix TestAttachmentDeleteOnExpiry flake

[torcolvin]

CBG-3838 make sure document gets fully imported before next step occurs.

The rare race condition for this test occurred when a document gets imported by import listener and then the on demand import got cancelled, but before the attachments are deleted. Attachments are deleted

sync_gateway/db/crud.go

Line 2220 in d764940

err = db.dataStore.Delete(previousAttachmentID)

but document is updated

sync_gateway/db/crud.go

Line 2121 in d764940

}