Convert test functions to IR

Currently we skip over test functions when building the IR for a module.

See: https://github.com/google/xls/blob/24046e424af962efd2b151d383cb9e01071bc1c8/xls/dslx/extract_conversion_order.cc#L431

This creates significant complications:
1. [Missing some parametrics in IR conversion](https://github.com/google/xls/blob/24046e424af962efd2b151d383cb9e01071bc1c8/xls/dslx/run_routines.cc#L62)
2. Hiding inconsistencies in IR handling (see #421)
3. Dropping some assertions and `fail!` calls before the JIT.

Especially now that we have [implicit assertion plumbing as part of IR conversion](https://github.com/google/xls/commit/e0f1bf95c4a8e07ca581d988dcfabb7b8c08f87f) we should convert test functions to resolve these issues.