DSLX-test JIT does not JIT the test fn bodies, only invoked functions

Closing this out, though going to create a derived issue for the (smaller task) remaining bits:

1. We don't translate the test function body itself (the one generally containing `assert_*` builtin calls), only function calls (via `Invocation` nodes) that are reached from the test function entrypoint.
2. We don't reuse JIT objects via our Python bindings; e.g. would be useful to amortize overhead in QuickCheck, we have to re-JIT every time at the moment I believe.

_Originally posted by @cdleary in https://github.com/google/xls/issues/10#issuecomment-682260404_