Attempt to reduce intermittent failures in the "multiple render() on the same canvas" unit-test
Attempt to reduce intermittent failures in the "multiple render() on the same canvas" unit-test

This patch should *hopefully* remove the intermittent unit-test failure, by using the *same* `optionalContentConfigPromise` for both `renderTask`s and thus get more predictable timing behaviour.

nawawi/pdf.js,timvandermeij/pdf.js,mozilla/pdf.js,Shoobx/pdf.js,showpad/mozilla-pdf.js,xavier114fch/pdf.js,Snuffleupagus/pdf.js,showpad/mozilla-pdf.js,nawawi/pdf.js,macroplant/pdf.js,caremerge/pdf.js,macroplant/pdf.js,xavier114fch/pdf.js,caremerge/pdf.js,timvandermeij/pdf.js,Shoobx/pdf.js,showpad/mozilla-pdf.js,Shoobx/pdf.js,Snuffleupagus/pdf.js,macroplant/pdf.js,timvandermeij/pdf.js,mozilla/pdf.js,mozilla/pdf.js,Snuffleupagus/pdf.js,nawawi/pdf.js