test(production-e2e): Add offline tests

[vtenfys]

Changes:

• Introduce CYPRESS_SUPPORT env variable for whether to enable Cypress API hooks, and when running tests, set this variable
• Use TEST_PLUGIN_OFFLINE env variable to determine whether or not to install the offline plugin when testing
• Add some additional tests which check the SW is installed correctly and perform a few things which I'd been manually testing
  • Existing production-runtime tests also now run both without and with gatsby-plugin-offline
• Update CircleCI config to allow running custom test commands
• Disable chromeWebSecurity for production-runtime tests because it doesn't work with service workers :(
• Delete some Cypress files accidentally added in the wrong place

[vtenfys]

Gatsbygram tests failed due to a segfault, not due to code changes here

[pieh]

Let's get this in - ideally we could adjust config a bit so it's more clear when production-runtime tests in offline or non-offline mode - right now there is single step ./scripts/e2e-test.sh "e2e-tests/production-runtime" "yarn test && yarn test:offline", but it's better to have those tests at all.

Thanks @davidbailey00!