Add integration test of a one-shot server client in a spawned process

[glyn]

@jdm pointed out that Servo spawns a process, rather than using fork(), to create a child process which acts as a client to a one-shot server in the parent process. It would be good to add an integration test to this repository to cover that usecase.

Background

A Rust internals thread Why no fork() in std::process? discusses some of the downsides of fork() and links to the excellent paper A fork() in the road, which describes why spawning a process is much safer than using fork().

The Servo code that spawns a process is here. (It sometimes uses servo/gaol as an alternative to spawning a completely new process, so perhaps it would be worth adding a testcase for that usecase too.)

[glyn]

To avoid duplication of effort: I'm looking to implement this.