pdf.js: Fix intermittent unit/reference test failures

To debug this, pull request #12124 included the browser name in the logs for the unit tests too.

The following is a list of unit test failures we have seen:

• TEST-UNEXPECTED-FAIL | cleans up document resources during rendering of page | in firefox | Unhandled promise rejection: Error: shall fail cleanup in http://127.0.0.1:38165/node_modules/systemjs/dist/system.js line 4 > eval (line 1585) (Linux; fixed in #12126)
• TEST-UNEXPECTED-FAIL | creates pdf doc from non-existent URL | in chrome | Unhandled promise rejection: MissingPDFException: Missing PDF "http://127.0.0.1:38165/test/pdfs/non-existent.pdf". (Linux)
• TEST-UNEXPECTED-FAIL | gets default page mode | in chrome | Unhandled promise rejection: AbortException: Worker was terminated. (Linux; fixed in #12144/#12136)
• TEST-UNEXPECTED-FAIL | gets non-existent outline | in chrome | Unhandled promise rejection: AbortException: Worker was terminated. (Linux; fixed in #12144/#12136)
• TEST-UNEXPECTED-FAIL | gets operatorList with JPEG image (issue 4888) | in chrome | Unhandled promise rejection: AbortError: The user aborted a request. (Windows; fixed in #12144/#12136)
• TEST-UNEXPECTED-FAIL | should correctly render PDFs in parallel | in firefox | Unhandled promise rejection: AbortError: The operation was aborted. (line 369) (Windows; fixed in #12144/#12136)
• TEST-UNEXPECTED-FAIL | multiple render() on the same canvas | in firefox | Failed: shall fail rendering (Windows; fixed in #12171)

It’s clear from this overview that most problems are related to unexpected action abortions, both in Chrome/Firefox and on Windows/Linux. This wasn’t happening a few days ago, so maybe something regressed? The second one is also interesting since we explicitly catch the exception in the unit test.

Pull request #12125 tries to improve the situation for the reference test failures.