dep graph: test and binary targets can be build in parallel

[matthiaskrgr]

cargo 7ab4778
I was looking at the timing of cargo-cache and it looks like creation of (binary,testrunner) and tests is not parallelized although it could be:

http://gistpreview.github.io/?5b6c67239b69c83b0cbaf1338f10d5f8

build command: cargo build --release -Ztimings --all-targets -j 20