Calling process::exit(1) from a proc macro looks successful to trybuild, but failure to cargo

```console
test tests/ui/error.rs ... error
Expected test case to fail to compile, but it succeeded.
```

Even though, compiling the same macro invocation directly using cargo, cargo sees that it failed and exits nonzero.

```console
$ cargo check
Checking repro v0.0.0
error: could not compile `repro` (bin "repro")

$ echo $?
101
```

Not sure whether there is any way to discern this from cargo's `--message-format=json` representation.