Improve module sandboxing and allow init code customization.

[maleadt]

• Instead of special-casing setup.jl, allow customization through a init_code kwarg.
• That code is now evaluated at the start of the sandbox module, instead of globally.
• This allows us to simply use include instead of having to rely on Main.include.
• Simplify test runner registration by using expressions rather than ugly and "expensive"-to-serialize functions.