為 args.gn 和 regen 新增 GN 標籤
usage: fx add-test //foo/bar //foo/baz
Adds the labels to `developer_test_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.