test_termination_exception seems to be flaky in CI

[tisba]

See https://github.com/rubyjs/mini_racer/actions/runs/24328353828/job/71028357479?pr=403

[…]
#<Thread:0x0000fffb8ad72e50 /home/runner/work/mini_racer/mini_racer/test/mini_racer_test.rb:1182 aborting> terminated with exception (report_on_exception is true):
MiniRacerTest#test_termination_exception = 1.51 s = .
/home/runner/work/mini_racer/mini_racer/test/mini_racer_test.rb:1182:in `heap_stats': exception class/object expected (TypeError)
	from /home/runner/work/mini_racer/mini_racer/test/mini_racer_test.rb:1182:in `block in test_termination_exception'
[…]

Last seen on linux-ubuntu-24.04-arm, ruby-3.2, musl. But I think I've seen it on other combinations too. Will keep an eye on it.