Allow turning on tests/benchmarks per package

In commercialhaskell/stack#917, it turns out it would be nice to be able to turn on tests (or benchmarks) selectively per package instead of for all of them at once. So when you do `cabal install --enable-tests foo bar baz`, it currently enables the test suites for all three packages. I'd want some way of specifying that I want to enable tests only for e.g. `foo`.