Add test for free_items_from_bins()

Federico Mena Quinterorequested to merge
federicomenaquintero/gga-cgt:bool-array into master

It fails like this:

---- sol::tests::free_items_from_bins stdout ----
thread 'sol::tests::free_items_from_bins' panicked at 'assertion failed: `(left == right)`
  left: `[Index(0), Index(1), Index(2), Index(3)]`,
 right: `[Index(4), Index(5), Index(6), Index(7), Index(8), Index(9), Index(10), Index(11), Index(12)]`', src/sol.rs:300:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Note how the original version was basically just a flatten(); the new version is the indexes that do not appear in the bins.

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