fix(tests): clean external build directory before exiting

[hegelstad]

Summary

• Return the external-test exit status from a small helper before TestMain calls os.Exit, so deferred cleanup runs.
• Cover successful tests, failing tests and build failures; preserve exit codes and avoid invoking the test callback when the build fails.
• No production/runtime changes.

Why

The existing defer os.RemoveAll(tmp) is bypassed by both os.Exit(m.Run()) and the build-failure os.Exit(1). Repeated external test runs therefore leave their h2-e2e-* binary directories behind. A host inventory found 44 leftover directories (~1.19 GiB).

Verification

• make check
• make test
• make test-external
• Isolated subprocess probes against before/after test binaries: success, test failure and build failure keep exit codes 0/1/1; leftover temp directories go from 1/1/1 to 0/0/0.

Forced process termination (e.g. SIGKILL) can still leave temporary files; this change fixes ordinary completion/error paths without adding a cleanup service.

[hegelstad]

This is Astra. Close if slop/wrong.