Fix occasional test failure due to env::set_var with multiple test threads

[spl]

Tests that modify environment variables (e.g. CFLAGS and CXXFLAGS) can cause tests to fail when run in parallel because they change a shared context.

This change moves the problematic tests into separate modules. Since each tests/*.rs is compiled as a separate crate, these tests are not run in parallel with other tests.

[alexcrichton]

Thanks! Could this also remove the --test-threads 1 argument we pass on CI?

[spl]

Could this also remove the --test-threads 1 argument we pass on CI?

This and #419 are the only problems I've encountered with parallel test threads. Unfortunately, the work I've been doing on cc-rs exacerbated these issues, which is why I've been chasing them down.

[alexcrichton]

It seems the intention of this PR is to basically enable cargo test though, right? If that's the case we should be able to run that on CI to verify it?

[spl]

I guess you're suggesting that I remove --test-threads 1 in CI. I don't think this PR is enough to allow that, since #419 is unresolved, but at least you can see the result now (1f81b89, build results).

[spl]

It seems like my hack (aa968a0) is not enough to “fix” #419 on Azure (build results).

[alexcrichton]

Ah ok, if that's the case mind if we hold off on this? Right now the test suite just needs to be run with one thread, but if there's more lurking then I'd prefer if it were all fixed instead of only having half of it fixed

[spl]

Alternatively, with d098048 implemented, it seems like you only need --test-threads 1 on macOS. Then, you only have one outstanding known issue with multiple test threads rather than two. In my view, I don't see the downside of merging this even if you don't remove --test-threads 1 from CI.

[spl]

Okay, I think I've fixed all the test threads issues. The failures in the build results appear to be network-related.

[spl]

Huh. I accidentally deleted the branch, which closed the PR. Then, I restored it and re-opened the PR. And that had the unexpected result of re-running the Azure build. 🤷‍♂