Investigate Haskell rewrite of testsuite driver

Currently, the testsuite driver is written in Python. This is a bit of a liability:

• Having to program in "not Haskell" might scare away potential contributors here
• Even developers who are proficient in Python probably prefer working with Haskell
• Managing Python as a dependency poses an additional burden and complicates deployment, setting up dev environments, CI, etc.; especially on Windows.
• We miss out on the benefits of type checks and typed programming for a substantial bit of infrastructure
• We miss out on a nice dog-fooding opportunity

So in order to judge the scope of this task, and define it better, it would be good to investigate a bit:

• What would it take to get high-level feature parity with the current solution?
• Does the Haskell ecosystem cover the concerns that we would like to address with existing libraries? (Process management, for example)
• Can we come up with a smart and frictionless way of migrating all the existing test cases in a reliable automated fashion?
• What are risks and unknowns?

#15363 (closed) is the recent case that sparked this - rather than the proposed patch there, which ports existing Haskell code to Python, we would prefer going in the other direction.

#17933 (closed) is another

Trac metadata

Trac field	Value
Version	8.4.3
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Test Suite
Test case
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