test: add a benchmark that is slow to elaborate (leanprover#5656)

Add an example Lean file that includes an unusually large definition
that takes a long time to elaborate.

It may be that it's difficult to process it more efficiently, but
perhaps someone will discover a way to improve it if it's in the
benchmark suite. Improved performance on this benchmark will likely make
some program analysis and verification tasks within Lean more feasible.

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Co-authored-by: Sebastian Ullrich <sebasti@nullri.ch>