There's a lot of history in this area that mostly explains why this
has not been supported in the past, leading to several awkward
workarounds in tests. But it doesn't appear that there's any reason
to prevent tests from creating actual command lines, so that's what we
allow here.
I've elected to flesh out `Cu.createCommandLine` rather than expose
`.init(..._)` directly since `Cu.createCommandLine` is already used in
the relevant tests and is only used in tests. And I've made the
arguments required, rather than optional, so that consumers think
about the arguments, in particular `state`.
Differential Revision: https://phabricator.services.mozilla.com/D132089
bac414edd4