Add separate script for nightly end-to-end tests

Currently, we only have a single set of end-to-end tests. All of which are being run via run-pre-commit-tests.sh for every Travis run.

We should add another script, run-nightly-tests.sh, which runs more heavyweight tests that we only want to run nightly or manually, for example when verifying a release.

We should:

• add the script
• document how to work with this in the README.md