Auto merge of #59769 - RalfJung:compiletest-normalization, r=alexcrichton

compiletest normalization: preserve non-JSON lines such as ICEs

Currently, every non-JSON line from stderr gets normalized away when compiletest normalizes the output. In particular, ICEs get normalized to the empty output. That does not seem desirable, so this changes normalization to preserve non-JSON lines instead.

Also see https://github.com/laumann/compiletest-rs/issues/169: because of that bug, Miri currently *looks* green in the toolstate, but some tests ICE. That same bug is likely no longer present in latest compiletest because the error code gets checked separately, but it still seems like a good idea to also make sure that ICEs are considered stderr output:
This change found an accidental user-visible `error!` in CTFE validation (fixed), and a non-deterministic panic when there are two `main` symbols (not fixed, no idea where this comes from). Both got missed before because non-JSON output got ignored.