testing: enable the Build Cop Bot

[tbpg]

Instructions here: https://github.com/googleapis/repo-automation-bots/tree/master/packages/buildcop

[tbpg]

I installed the bot on the repo (first step of the instructions). We still need to call the buildcop binary to publish the test logs.

[fhinkel]

We need to update all the tests to generate xUnit xml output before we can turn on the bot.

[tbpg]

Snippet I've seen elsewhere:

npm test -- --reporter=xunit --reporter-option='output=sponge_log.xml'