skyfocus: add test for directories in working set.

Due to how Skyframe and Skyfocus already works, defining directories in
`--experimental_working_set` automatically keeps the `FILE_STATE` SkyKeys of the files in those directories for focusing. Take this rdep graph for example:

`FILE_STATE:[dir] -> FILE:[dir] -> FILE:[dir/BUILD], FILE:[dir/file.txt] -> .. -> BUILD_DRIVER[..]`

If we focus on `dir/`, then Skyfocus will keep these above rdeps, and also the
`FILE_STATE` SkyKeys of the individual files, because `FILE` depends on `FILE_STATE`. This allows the diff invalidators
in `SkyframeExecutor#handleDiff` to correctly invalidate those `FILE_STATE` SkyKeys should
their associated files change.

RELNOTES:
PiperOrigin-RevId: 609694514
Change-Id: I1ea0de36b47e4151045df1d3f7f82807d8ea312f