Tests fail on macOS

[usize]

Summary

Running make test on macOS fails due to several platform incompatibilities:

• sha256sum not available: macOS does not ship GNU coreutils; the Makefile's tool verification depends on sha256sum
• h2spec lacks arm64 builds: Only amd64 binaries are published, requiring Rosetta translation on Apple Silicon
• Networking behavior differences: Socket timing and ephemeral port behavior differ between macOS and Linux, causing intermittent test failures
• BusyBox flag incompatibility: The Makefile uses sha256sum --check --status (GNU long flags) which are not portable

Desired outcome

Investigate fixing the tests to run natively on macOS, or at minimum document which tests are expected to fail and why.

[esnible]

Everything works on Mac except for the certificate rotation test, and #71 will disable it for macOS tests.