Support running of `libtest` style (unit-test) benchmarks

[Daniel Müller]

Would be great to support unit-test style benchmarks ala libtest, i.e., the nightly stuff. See https://doc.rust-lang.org/1.4.0/book/benchmark-tests.html

I haven't found a way to invoke cargo-flamegraph that would allow for running them, but do let me know if that's already possible, please.