flaky tests: async after open

[lukekarrys]

The write tests that wait for open before writing to the stream sometimes have missing data from the file after the finish event:

These two tests have been flaky across node versions and platforms:

https://github.com/npm/fs-minipass/blob/54a6a0d2bb5e4f8a4949180ab14e2ded5a9c20d2/test/write.js#L103

https://github.com/npm/fs-minipass/blob/54a6a0d2bb5e4f8a4949180ab14e2ded5a9c20d2/test/write.js#L121

After initial triage with @nlf, we think the culprit might be the _flush method being called more than once synchronously without the fs.writev callback finishing.

https://github.com/npm/fs-minipass/blob/54a6a0d2bb5e4f8a4949180ab14e2ded5a9c20d2/lib/index.js#L372-L373