Tool support for testing wasm targets

[brson]

In order to make the wasm targets production ready we'll need a testing solution. Right now I'm inclined to teach cross how to do it and call that the solution, but that excludes macOS hosts, and for now at least, Windows as well, since it relies on docker.

cc @japaric

[japaric]

Right now I'm inclined to teach cross

See rust-lang/cross#36. If rustc emitted executable .js files with a #!/usr/bin/env node shebang in them then cross test would work for the asmjs target. I'm not sure what else would need to be done for the wasm target though because node hello-wasm.js errors with node v7.3.0 as stated in that PR.

[alexcrichton]

I believe we've since implemented the "runner" configuration in Cargo, which I believe solves this.