Track memory usage in tests

[macournoyer]

How do we make sure that:

• there are no memory leaks in our code
• everything that needs to be GCed by Ruby has been
• we don't use Ruby objects that have been GCed (GC.stress=trye could do the trick, but is slow, see https://github.com/Shopify/liquid-wasm/issues/168. It could be an option we turn on in CI?).

I think we could leverage #18, and measure memory usage during the execution. After a warmup phase, memory usage should be stable.

Maybe we could also measure allocations like in SFR.