WIP: [ffigen] Trying to fix global_test flake

[liamappelbe]

global_test and global_native_test are exactly the same, except that one uses ordinary bindings, and the other uses @Native style bindings. When both tests are run in the same suite, occasionally they will both fail. It's always the case that both fail or both pass.

The flake appears to be in the build, not the test. If I change the workflow to build once and run the tests 10 times (and count the failures instead of stopping on failure), then it's always the case that there are either 0 or 10 failures: https://github.com/dart-lang/native/actions/runs/11471378290/job/31980751070?pr=1674

I hit a bit of a wall at this point, because I'm not sure how to get the bad build artifacts from github. So I've just changed these two tests to share the same native code. They still have their own ffigen bindings, but the underlying so file only has one copy of the global variables. Just have to make sure to reset the test state after each test, so they don't interfere with each other.

This seems to have fixed the flake, because there's no longer a conflict between the tests native artifacts. To verify the fix I changed the workflow to run the build/test 10 times, and there were no flakes: https://github.com/dart-lang/native/actions/runs/12386195112/job/34573726773

Still flaky.

[coveralls]

coverage: 87.958%. remained the same
when pulling 02de189 on more_global_flake
into f2a0e28 on main.

[liamappelbe]

Of course, as soon as I put this PR out for review, it starts flaking again 😩