Upgrades puppeteer, which uncovered a slew of timing errors in Perspective's visual-regression test suite. To help rectify this:

• perspective-viewer flush() and internal _update() methods now use a async-aware debounce method borrowed from regular-table, modified to allow settings the "updating" flag only when this debounced method is idle.
• perspective-workspace now calls MessageLoop.flush() in a number of places where perspective would expect side effects to either live in a Promise or happen synchronously.
• Several tests have their conditions streamlined.
• Modified azure-pipelines.yml to publish screenshots when JavaScript tests fail. This is only partially useful since there are no reference screenshots to diff on CI, but it is better than nothing.