Added run_flags directive to `compiletest`

Now it is possible to specify run-flags in tests. For example, by using `run-flags: --bench` the Bencher is run.
auto merge of #13553 : aochagavia/rust/pr, r=alexcrichton

Now it is possible to specify run-flags in tests. For example, by using `run-flags: --bench` the Bencher is run.