rustc: Don't continue to resolve if there's an error in lang_items

This confused me when it happened.
rustpkg: Write more automated tests

Automate more tests described in the commands.txt file,
and add infrastructure for running them. Right now, tests shell
out to call rustpkg. This is not ideal.
auto merge of #7148 : catamorphism/rust/rustpkg_tests, r=graydon

r? @graydon Automate more tests described in the commands.txt file,
and add infrastructure for running them. Right now, tests shell
out to call rustpkg. This is not ideal.

Goes part of the way towards addressing #5683