This patch:
- changes Puppeteer-Firefox plumbing of defaultBrowserOptions to align
with the way we do it for Puppeteer.
- plumbs puppeeteer-dependent Errors and DeviceDescriptors down to every
test.
- unifies a few tests between Puppeteer-Firefox and Puppeteer.
**Note:** in future, we should expose errors as `puppeteer.errors` and
device descriptors as `puppeteer.devices` to make it easy to pass around
Puppeteer/Puppeteer-Firefox instance.
References #3889.
