Convert test functions to IR

[nanavati]

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

See:

xls/xls/dslx/extract_conversion_order.cc

Line 431 in 24046e4

// We don't convert the bodies of test constructs of IR.

This creates significant complications:

1. Missing some parametrics in IR conversion
2. Hiding inconsistencies in IR handling (see Consistently support compound equality in the IR #421)
3. Dropping some assertions and fail! calls before the JIT.

Especially now that we have implicit assertion plumbing as part of IR conversion we should convert test functions to resolve these issues.