Expanding the test infrastructure for a "fancy suite"

[alexcrichton]

Right now we have the ability to test rust programs that consist of entirely rust source code through the various src/test/XXX suites. I've been thinking recently that we may want a "fancy suite" which supports lots of custom-building options like mixing C/C++ with rust in various flavorful ways. Some examples of this would be:

• Printing "hello world" from a rust library which is bootstrapped from C
  • This would show a basic usage of bootstrapping the runtime externally
• Exporting a symbol to get used from C at various locations in a library
• Calling back and forth between C and Rust to make sure that things work out OK
• Possibly other flavorful things with #[no_std] and friends (but perhaps limited support for I/O).

I was thinking something along the lines of src/test/fancy-suite/testname/Makefile would basically be the "run this test" rules. Each test would be responsible for doing its own validation of output/errors/whatnot.

I'm not sure if this really warrants a whole new style of testing infrastructure, but some of the use cases that'd be nice to test I don't know how to test today. Although perhaps in theory they could all be written in just a rust program in src/test/run-pass, but the makefiles/such will probably need defines for the rustc and friends to use.

[catamorphism]

+1 -- just improving how cross-crate tests work would be a plus. I find the current way (aux-build) pretty awkward and always have to look up how it works :-)