test: use spawn() in pause/resume tests to avoid thread race

[congwang-mk]

Summary

• The TestPauseResume tests previously launched sb.run(...) in a worker thread and relied on time.sleep(0.15) / time.sleep(0.1) to wait for the sandbox to start before calling pause() / reading pid. On a slow CI runner this flaked with RuntimeError: sandbox is not running, because sandlock_create() hadn't yet returned and self._handle was still None.
• Rewrite both tests to use sb.spawn(), which is synchronous and guarantees pid is set on return. No worker thread, no polling, no race.
• Drop the now-unused threading import.

Test plan

• python3 -m pytest python/tests/test_sandbox.py::TestPauseResume -v (4 passed)
• python3 -m pytest python/tests/ (247 passed)