[infra][ffigen] `test/setup.dart` failures don't fail the CI

Take output from [this job](https://github.com/dart-lang/native/actions/runs/10461232655/job/29098801644)

```console
Run dart test/setup.dart

Running build_test_dylib.dart in D:\a\native\native\pkgs\ffigen\test\native_test\
Building Dynamic Library for Native Tests...
native_test.def : error LNK2001: unresolved external symbol Enum1
native_test.def : error LNK2001: unresolved external symbol Enum2
native_test.def : error LNK2001: unresolved external symbol StructWithEnums
native_test.lib : fatal error LNK[11](https://github.com/dart-lang/native/actions/runs/10461232655/job/29098801644#step:5:12)20: 3 unresolved externals

clang: error: linker command failed with exit code 1[12](https://github.com/dart-lang/native/actions/runs/10461232655/job/29098801644#step:5:13)0 (use -v to see invocation)

Success :)
```

Which leads to all the tests running, then the native test failing:
```console
❌ test\native_test\native_test.dart: native_test (setUpAll) (failed)
The file test\native_test\native_test.dll does not exist.

You may need to run: dart run test/setup.dart

test\test_utils.dart 43:5 verifySetupFile
test\native_test\native_test.dart 28:7 main..
```

Not a big deal imo, but it would probably be cleaner to fail early