Remove need for delays when testing neovim jobcontrol

In #234, `(0.2s)` delay controls were added to `CallAsync` tests in system-job.vroom. Figure out why that extra delay is needed, fix the cause, and remove the delays.

Some notes:
- The commands don't seem to need the extra delay in practice (plugins depending on the jobcontrol implementation use it and work fine)
- Per [debugging detailed in this comment](https://github.com/google/vim-maktaba/pull/234#issuecomment-668669030), with no delay, it appears that `maktaba#syscall#neovim#HandleJobExit` is receiving the stdout of the call, but shell.vroomfaker isn't reporting that it received it.