This patch:
- stops appending `undefined` to our protocol messages unnecessarily.
- rewrites `Cannot find execution context id` to `Execution context was destroyed, most likely because of a navigation.` when it occurs from a Puppeteer ExecutionContext. The error message is left alone if it occurs via a CDPSession.
Today, we have tests split into multiple files, with files pulling
tests from some other files.
This patch starts explicitly gathering all tests from the same
`test.js` file.
Drive-By: move one test from `browser.spec.js` into `puppeteer.spec.js`
since it starts browser itself.