expose test binary path

[danburkert]

There should be a way to get the path to the binary resulting from cargo test --no-run via scripts in a non interactive setting.

[danburkert]

It can be done via file globbing, but it's a little messy.

[alexcrichton]

My preferred method of doing this would be to actually just have predictable names, so I'm going to close this in favor of #1924, but thanks for the report!