add a GN label to args.gn and regen

    usage: fx add-test [--target-list TARGET_LIST] //foo/bar //foo/baz

    Adds the labels to the given TARGET_LIST and regenerates. If --target-list is not
    specified, the target is developer_test_labels.

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