Move spawnAndWait(forExecutableAtPath:) to a separate file.

[grynspan]

Bookkeeping—move that function to its own file to make it easier to read exit tests' code.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

This is just moving code from one file to another, so I'll self-review.

[grynspan]

@swift-ci test