fx 新增測試

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.