tests: fix clippy::zombie_processes finding

[cpu]

Nightly clippy's clippy::zombie_processes lint flagged the following:

error: spawned process is never `wait()`ed on
   --> tests/client_server.rs:285:26
    |
285 |         let mut server = self.server_opts.run_server();
    |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: consider calling `.wait()`
    = note: not doing so might leave behind zombie processes
    = note: see https://doc.rust-lang.org/stable/std/process/struct.Child.html#warning
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#zombie_processes

While we do call kill() on the process, we weren't wait()ing it. The process::Child docs have a warning:

On some systems, calling wait or similar is necessary for the OS to
release resources. A process that terminated but has not been waited on
is still around as a “zombie”. Leaving too many zombies around may
exhaust global resources (for example process IDs).

So it seems it may not be sufficient on all systems to kill() without wait().

Let's add a wait() just to be sure. Nobody likes zombies.