BigInt is slow on most targets (or at least tests are)

See #12309

The overhead is not negligible, with total CI times being multiplied by 3-4 on some targets. There are a lot of tests involved, but seeing how hl and jvm are still fine with it, it should be possible to have them run in a decent amount of time on other targets as well.

Start with issue #12309 and the BigInt-related tests, then compare CI timings across targets where hl and jvm are already acceptable. Identify which target test runs cause the 3–4x slowdown; done means the affected targets complete in a reasonable time without losing BigInt coverage.

Written by the indexing model from the issue text.