The other testing code already uses the name of the browser as the
unique identifier, so I don't see a good reason to not use that for
identifying browsers to quit as well. Doing so simplifies the (already
somewhat complex) testing logic and ensures that we can use existing
functionality (such as the `getSession` function) to retrieve sessions.

This commit replaces our own infrastructure for handling browsers during
tests with Puppeteer. Using our own infrastructure for this had a few
downsides:

- It has proven to not always be reliable, especially when closing the
  browser, causing failures on the bots because browsers were still
  running even though they should have been stopped. Puppeteer should do
  a better job with this because it uses the browser's test built-in
  instrumentation tools for this (the devtools protocol) which our code
  didn't. This also means that we don't have to pass
  parameters/preferences to tweak browser behavior anymore.
- It requires the browsers under test to be installed on the system,
  whereas Puppeteer downloads the browsers before the test. This means
  that setup is much easier (no more manual installations and browser
  manifest files) as well as testing with different browser versions
  (since they can be provisioned on demand). Moreover, this ensures that
  contributors always run the tests in both Firefox and Chrome,
  regardless of which browsers they have installed locally.
- It's all code we have to maintain, so Puppeteer abstracts away how the
  browsers start/stop for us so we don't have to keep that code.

By default, Puppeteer only installs one browser during installation,
hence the need for a post-install script to install the second browser.
This requires `cross-env` to make passing the environment variable work
on both Linux and Windows.

To save time or resources during development it can be useful to run
tests only in Firefox. Previously this could be done by editing the
browser manifest file, but since that file is no longer used for
Puppeteer, this command line flag replaces it. For example, executing
`gulp unittest --noChrome` will only run the unit tests in Firefox.