A test suite for new backends

Stream: lean4 dev

[Siddharth Bhat]

I've written two backends for Lean so far (a prior MLIR based backend, and the current LLVM backend). Over the course of this, I developed a test suite that tests each compiler feature in isolation: https://github.com/bollu/lean4/tree/1-aug-2022-llvm-backend/tests/backendramp

I propose upstreaming this test suite, because it would provide a logical place to test new features to the backend, and help in the development of alternate backends. This test suite would concretely help with the current LLVM size_t fixes, and might help provide minimal test cases for the ongoing WASM effort.