Allow turning on tests/benchmarks per package

[hesselink]

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.

[ttuegel]

We should allow --enable-tests to take an optional list of packages so that --enable-tests=foo,bar only enables tests for foo and bar, but --enable-tests continues to enable tests for all explicit targets.

[grayjay]

I think this is possible with cabal install foo bar baz --constraint='foo test'.

[hvr]

Btw, this is also possible already via cabal.project files by using package-stanzas, e.g.

package foo
  tests: true
  benchmarks: true

package bar
  tests: false
  benchmarks: true

What's missing though is a convenient ability to enable/disable individual tests within a package when there's multiple testsuites defined in a package.