add a GN label to args.gn and regen

    usage: fx add-host-test //foo/bar //foo/baz

    Adds the labels to `host_labels` and regen.
    This is useful when `fx test` tells you to add a target to the build,
    but you have some custom args set in `args.gn`, so do not want to run
    `fx set` to overwrite those args.

[add-host-test source code](https://cs.opensource.google/fuchsia/fuchsia/+/main:tools/devshell/contrib/add-host-test)