Get unit tests for rusti working

* They didn't work before, because the location of the tests caused the
  'sysroot' option to crate lookup to be wrong for finding the correct stage's
  core/std libraries. This moves the compiled tests from the $host/test
  directory into a $host/$stage/test directory. This means that the sysroot will
  be correct and the core/std libraries can actually be found
* The LLVM bindings apparently aren't threadsafe, so we can't run multiple tests
  in parallel.
Fix `use` statements with rusti
Add regression tests for various other rusti issues
Don't emit common warnings in rusti
auto merge of #6467 : alexcrichton/rust/better-rusti, r=thestinger

These few commits address a few existing issues:

* #5469 - adding regression tests for `rusti`. This adds unit tests to the `rusti.rc` file (which needed some reorganization of the Makefile, see the first commit message). These are super-simple right now, and sadly can't test the output of the tests. I worked for a bit on making a compiletest version of the rusti tests, but I ended up hitting something which blocked me, although I've forgotten it by this point.
* #5937 - regression test added, and it's fixed
* #5803 - just doesn't appear to happen any more
* #5784 - it's no longer broken, and it no longer spits out warnings about unused variables.

I also did some investigation into #5774, and you may want to read the comment I left on the bug. The gist of the situation is that C++ exceptions across JIT code don't look like they're working, even though they [should be working](https://github.com/mozilla/rust/blob/3aa1122ec25d15a2a73a295f8298ad9c38b09a10/src/rustllvm/RustWrapper.cpp#L387). If anyone has any insight on this, that would be awesome!