Allow testing of interactivate code paths within BuildStream

The following discussion from !662 (merged) should be addressed:

@Qinusty started a discussion:
I have removed the tests I was prototyping from this branch.

Further work is required for testing interactive parts of BuildStream

Tasks:
- Be able to cli.run() on a separate process.
- Be able to send signals to such a process.

BuildStream test utils currently provide no utilities for running BuildStream on a separate process. This inhibits the ability to test BuildStream's interactive code paths.

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