Commit 052079c99455d01312d377d72fa1b8b5c0b22aad surfaced two issues with
the generated C code:

  - renderInt128() contained a seemingly unnecessary assertion to verify
    that the high 64 bits of the number were nonzero, dating back to
    9bf1681990fe87a6b2e5fc644a89f1aece304579. I removed it.
  - renderValue() didn't have any special handling for undefined structs,
    falling back to printing "{}" which generated invalid expressions
    such as "return {}" for functions returning structs, whereas
    "return (S){}" is the correct form. I changed it accordingly.

At the same time I'm reenabling the relevant tests.