test: do not leave dangling listener on shared browser object (#1674)
test: do not leave dangling listener on shared browser object (#1674)

This resulted in quite a few failures on travis.
GoogleChrome/puppeteer,puppeteer/puppeteer,puppeteer/puppeteer,puppeteer/puppeteer,puppeteer/puppeteer,GoogleChrome/puppeteer,GoogleChrome/puppeteer