Add --enable-{tests,benchmarks} to freeze command

This MR adds the `--enable-tests` and `--enable-benchmarks` options to `cabal freeze`, as requested in #1827. If present they will cause the dependencies of all test and benchmark stanzas to be included in the set of frozen dependencies.

The desire for these flags is perhaps an indicator that `freeze` should be configuring the package too. In which case either the full set or some subset of the configure flags should be made available to `freeze`. I choose not to go that route at the moment as it seems like a lot of work for gains which are uncertain, at least to me.