Add a basic CI test checking that Emacs loads normally

[bbatsov]

Such a test would be very useful on CI to prevent from silly mistakes like the one in #1279 creeping in. Basically we need to:

• clone Prelude
• start Emacs

[hut8]

@bbatsov did you see the PR for this? No rush, just making sure.

[hut8]

This can be closed, right?

[bbatsov]

We now have a CI load test (test/prelude-load-test.el) that loads core and all modules in batch mode, running on multiple Emacs versions.