Add something like `cargo test --no-fail-fast` across all rustc tests

[cuviper]

It would be nice to have a way in rustbuild to run ALL testsuites, even after a failure is encountered. For instance, on Fedora I know some of the alternate architectures aren't 100% passing, so I don't gate our builds on this, but I would still like to get a complete picture of the failures.

With the makefiles, I was using something like this to plow through and manually get a summary:

make check -k || python2 src/etc/check-summary tmp/*.log