Move E2E panic tests into their own dedicated folder so mocha won't
automatically attempt to run it. New npm script (`npm run e2e`) runs
those tests with mocha explicitly.
This solves a number of problems:
- Travis-CI can't run the panic tests without a selenium server, which
requires a ton more setup that simply hasn't been done yet.
- Selenium-webdriver doesn't support node < v4, and immediately fails
the build on those environments.
If at any point in the future we want those tests to run automatically,
here's what we'll need:
- Browserstack or SauceLabs tunneling and auth env variables.
- Bash script to detect node version and only install deps and run the
tests if the environment supports it.
Also, a number of tests had `.only` in front, preventing all the others
from running. They've been removed.
