Add rusti/rustpkg to check-lite

Ironically also disable rusti tests for now, but mainly because they're awaiting
LLVM upgrades before proceeding
auto merge of #7568 : alexcrichton/rust/check-lite, r=huonw

A lot of cross-platform issues stem from rusti/rustpkg, so include these two test suites in the 'check-lite' target which is run on the cross-compile bots. It shouldn't be much of a performance hit because these suites are pretty fast to run.

Hopefully this will make snapshot/tarball creation easier in the future.