Run thread-heavy integration tests in serial

[ludfjig]

Tries to fix some recent test hangs that shows up in CI due to thread congestion and interrupt_infinite_loop_stress_test sleeping for 1000ms not being enough for a sandbox on another thread to enter a guest call. (see here for example)

If this doesn't fix it, we can add the attribute to some more thread-heavy tests in sandbox_host_tests, or introduce a barrier in a host call, but hopefully this is sufficient

[Copilot]

Pull request overview

This PR aims to reduce CI hangs by preventing the most thread-heavy integration tests from running concurrently and by adjusting timing in an interrupt-related stress test.

Changes:

• Marked several thread-heavy interrupt stress tests to run serially via serial_test (thread_heavy group).
• Increased the wait duration before sending an interrupt in interrupt_custom_signal_no_and_retry_delay and asserted kill() succeeds.
• Simplified the Justfile integration-test invocation by removing the special-case execution of interrupt_random_kill_stress_test.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/hyperlight_host/tests/integration_test.rs	Adds serial_test and serializes selected thread-heavy interrupt stress tests; tweaks interrupt timing/assertion in one test.
Justfile	Runs integration tests in a single invocation instead of skipping/rerunning one stress test separately.