Replace XMLHttpRequest usage with the Fetch API in `send` (in `test/unit/testreporter.js`)
Replace XMLHttpRequest usage with the Fetch API in `send` (in `test/unit/testreporter.js`)

Besides converting the `send` function to use the Fetch API, this patch also changes the method to return a `Promise` to get rid of the callback function. (Although, currently there's no call-site passing in a callback function.)

Snuffleupagus/pdf.js,nawawi/pdf.js,showpad/mozilla-pdf.js,mozilla/pdf.js,mozilla/pdf.js,timvandermeij/pdf.js,mozilla/pdf.js,showpad/mozilla-pdf.js,nawawi/pdf.js,Shoobx/pdf.js,Snuffleupagus/pdf.js,Snuffleupagus/pdf.js,showpad/mozilla-pdf.js,timvandermeij/pdf.js,macroplant/pdf.js,nawawi/pdf.js,Shoobx/pdf.js,Shoobx/pdf.js,timvandermeij/pdf.js,macroplant/pdf.js,macroplant/pdf.js