Problem:
- `n.spawn()` is misleading because it also connects RPC, it's not just
"spawning" a process.
- It's confusing that `n.spawn()` and `n.spawn_argv()` are separate.
Solution:
- Replace `n.spawn()`/`n.spawn_argv()` with a single function `n.new_session()`.
This name aligns with the existing functions `n.set_session`/`n.get_session`.
- Note: removes direct handling of `prepend_argv`, but I doubt that was
important or intentional. If callers want to control use of `prepend_argv`
then we should add a new flag to `test.session.Opts`.
- Move `keep` to first parameter of `n.new_session()`.
- Add a `merge` flag to `test.session.Opts`
- Mark `_new_argv()` as private. Test should use clear/new_session/spawn_wait
instead.
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