Flacky integration test

[dianabarsan]

Integration tests sometimes fail with messages similar (but not restricted to):

[2021-04-14T05:51:39.710Z ERROR]:  exiting
      1) should handle documents with \u0000 in it
[2021-04-14T05:51:39.733Z ERROR]:  An unrecoverable error occurred
[2021-04-14T05:51:39.734Z ERROR]:  error: index "couchdb_doc_attachments" does not exist
    at Connection.parseE (/home/runner/work/couch2pg/couch2pg/node_modules/pg/lib/connection.js:539:11)
    at Connection.parseMessage (/home/runner/work/couch2pg/couch2pg/node_modules/pg/lib/connection.js:366:17)
    at Socket.<anonymous> (/home/runner/work/couch2pg/couch2pg/node_modules/pg/lib/connection.js:105:22)
    at Socket.emit (events.js:198:13)
    at addChunk (_stream_readable.js:288:12)
    at readableAddChunk (_stream_readable.js:269:11)
    at Socket.Readable.push (_stream_readable.js:224:10)
    at TCP.onStreamRead [as onread] (internal/stream_base_commons.js:94:17)

Other indexes failing can be reported, like couchdb_doc_uuid already existing.
It looks like there's a race condition where the we're not actually waiting for the migrations to run before starting other migrations or starting testing.

I've seen builds fail on node 8 and node 10, so it doesn't seem to be related to node or package versions.