`nsIWebProgressListener.onStateChange()` gets called twice at the end of
requests, once with `stateFlags` set to `327696` and once with it set to
`262160`, which corresponds to `STATE_STOP + STATE_IS_NETWORK +
STATE_IS_REQUEST` and `STATE_STOP + STATE_IS_NETWORK`. httpd.js debug
logging shows that the connection is closed between the two calls. In
WebProgressFinishListener, we were previously calling `onFinish` after
the first one, but in Zotero 7, at least on Linux (or maybe just on
slower machines due to a race condition), the file from `saveURI()`
doesn't appear to be reliably written after the first call, causing
`Attachments.downloadFile()` to fail in `_enforcePDF()` due to an empty
file.
This changes WebProgressFinishListener to wait until the second
`STATE_STOP` call. We'll have to confirm whether this is the
state-change pattern for all requests, but it fixes our Find Available
PDF tests in CI.
febc2fcc7d